Diving Glossary

Small, brightly coloured reef fish that form dense clouds on Red Sea reefs. The orange-pink shimmer in every Red Sea photo is mostly anthias — specifically Pseudanthias squamipinnis, the most common species. They feed on plankton in the current and reorganise into smaller groups when threatened.

Anthias schools indicate healthy reefs with good current and food supply. Sites like Police Station and Umm Gamar are famous for their anthias clouds.

The PADI certification level above Open Water, allowing dives to 30 metres. AOW divers complete five additional adventure dives including a deep dive and underwater navigation dive.

In the Red Sea, AOW is the practical minimum for sites like the Thistlegorm wreck, El Mina, Salem Express, and most of the Brothers/Daedalus liveaboard destinations.

See also: OWD, PADI

The most famous (and dangerous) feature of Dahab's Blue Hole — a 26-metre-long natural tunnel connecting the inside of the sinkhole to the open sea. The top of the Arch sits at 55 metres depth.

Diving through the Arch requires advanced technical training, mixed gas, and proper planning. It is responsible for the majority of Blue Hole fatalities and should not be attempted by recreational divers under any circumstances.

See also: Blue Hole, Technical diving

Diver shorthand for the impossibly clear, deep-blue water that makes the Red Sea famous. Used in dive logs to describe visibility 30m+ and bright midday sun penetration.

The inflatable jacket-like vest a diver wears. Its main job is to control buoyancy — divers add air to ascend or rise off the bottom, release air to descend or sink. Modern BCDs also hold the tank, weights, and equipment.

BCD failures (most commonly stuck inflator buttons or runaway free-flow) are a leading cause of recreational diving accidents. Pre-dive checks always include testing both inflate and deflate buttons.

A natural underwater sinkhole. The most famous Red Sea Blue Hole is in Dahab — a 100m-deep, 30m-wide vertical sinkhole right at the shore, connected to the open sea via a deep underwater tunnel called the Arch.

The Dahab Blue Hole has earned the nickname "the divers' cemetery" due to over 130 deaths there since 1997. Almost all fatalities involve untrained recreational divers attempting the Arch on a single tank, or technical divers exceeding their gas limits. The recreational route — entering at El Bells and exiting via the saddle into the Blue Hole at 7m — is no more dangerous than any other Red Sea dive.

See also: The Arch, El Bells, Dahab Blue Hole & Canyon guide

An entry point about 200m north of the Dahab Blue Hole. A narrow vertical chimney in the reef table that drops to about 28-30 metres, named for the bell-like sound dive tanks make when scraping its walls.

The standard Blue Hole dive starts with a giant stride into El Bells, descends to 28m via the chimney, then drifts south along the wall to the Blue Hole's saddle at 7m. This is the safe, recreational version of "diving the Blue Hole" — and it's stunning.

The total time spent underwater on a dive, from descent to start of ascent. Used in conjunction with depth to calculate residual nitrogen and decompression status. Bottom time is one of the two key inputs (with depth) for dive tables and computers.

The pre-dive equipment verification routine. PADI uses the mnemonic BWRAF: BCD, Weights, Releases, Air, Final OK. Conducted between dive partners before entering the water.

Skipping the buddy check is one of the most common contributors to dive accidents. Take 30 seconds — it costs nothing and prevents real problems.

A famous Dahab dive site featuring an underwater fissure that opens at 18m and descends to 30m, with a narrow continuation deeper. Light enters from above through cracks in the reef, creating dramatic shafts of blue.

The Canyon is typically combined with the Blue Hole as a two-dive day trip. Requires AOW certification due to depth.

An emergency procedure for ascending to the surface after running out of air. The diver swims up while exhaling continuously to prevent lung overexpansion injury. Practised in Open Water training but should never be needed on a properly planned dive.

A short, easy first dive that lets your guide confirm your buoyancy, weighting and air consumption before you join a group on deeper or busier sites. Most Red Sea centres ask certified divers who haven’t dived in a while to do one — it’s a comfort and safety check, not a test.

At Aquarius the check dive is usually run on the house reef or a sheltered bay, so you can fine-tune your weights and trim in calm, shallow water.

A fixed location on a reef where small "cleaner" fish — mostly blue-streak cleaner wrasse — remove parasites from larger fish. Bigger fish queue up at these stations, opening their gills and mouths to be serviced.

Cleaning stations are reef-health indicators. Don't disturb them — divers blocking access can disrupt the entire reef ecosystem. Watch from a respectful distance and you'll see species you'd never get close to otherwise.

When corals are stressed — most often by unusually warm water — they expel the symbiotic algae (zooxanthellae) that give them their colour and most of their food, turning bone-white. Bleached coral is not yet dead and can recover if conditions ease, but prolonged stress kills it.

The northern Red Sea’s corals are unusually heat-tolerant, which is why these reefs remain among the most resilient on Earth — though divers still help by reporting bleaching and never touching the reef.

Hard (stony) corals build the reef’s limestone skeleton — the table, brain and branching formations you swim over. Soft corals have no rigid skeleton and sway in the current as colourful “trees”, spectacular on Red Sea sites like the Bells and Small Crack.

Both are colonial animals, not plants; together they create the living structure the whole reef depends on.

Water moving in a particular direction. The Red Sea has predictable seasonal currents but local conditions vary — strong currents at sites like Carless Reef, Elphinstone, and Brothers Islands are normal. Divers describe currents as "ripping" (very strong), "humping" (strong), "drift" (moderate), or "slack" (weak/none).

A good guide checks current direction before splashing and adjusts the dive plan accordingly.

See also: Drift dive

Standard day-trip diving format used across Egyptian Red Sea operators. Pickup from hotel, boat trip to dive sites, two dives with surface interval, lunch on board, return by mid-afternoon. Distinct from liveaboard trips which span multiple days.

An injury caused by dissolved nitrogen forming bubbles in the body when a diver ascends too fast or stays too long at depth. Symptoms range from joint pain and rash to dizziness and numbness. It is prevented by staying within no-decompression limits (see NDL), ascending slowly and making a safety stop.

DCS is treated in a recompression chamber; Hurghada has one. It is distinct from DECO, which refers to the planned decompression obligation itself.

Short for "decompression stops" — mandatory pauses during ascent that allow dissolved nitrogen to safely leave a diver's tissues. Required when a dive exceeds the No-Decompression Limit (NDL). Recreational diving is planned to stay within NDL — divers who go into "deco" without proper training and equipment risk decompression sickness ("the bends").

See also: NDL, Safety stop

The British WWII programme that placed naval gunners and weapons on merchant vessels. Five DEMS gunners died on the SS Thistlegorm — they manned the anti-aircraft guns when the bombers attacked.

DEMS gunners are remembered separately from the merchant sailors because they were Royal Navy personnel, not merchant seamen. Their cap badges and uniforms differed.

See also: Thistlegorm guide

A guided first breath underwater for people with no certification — no licence needed. After a short briefing on a few basic skills, a PADI instructor takes you into shallow water and often on to a real reef. It is the most popular way to find out whether diving is for you.

At Aquarius the Try Dive runs in calm, shallow conditions with close instructor supervision, and can be credited toward a full Open Water course later.

A wrist or console device that tracks your depth and time and continuously calculates your remaining no-decompression limit, ascent rate and safety stop. It has largely replaced printed dive tables and lets each diver follow their own profile.

Rental computers are available; if you own one, bring it — your guide will still set the dive’s maximum depth and time for the group.

A record of each dive — site, depth, time, conditions, air used and what you saw — signed by your guide or instructor. It documents your experience for dive centres and is required to enrol on most continuing-education courses.

Bring your logbook (or app) to the Red Sea; centres like to see recent dives, and there are plenty of wrecks and reefs here worth logging.

The first professional PADI rating. A divemaster can guide certified divers, assist instructors and supervise activities. Earning it takes solid experience and a focused course, and it is the gateway to becoming an instructor.

Aquarius runs the full PADI professional pathway in the Red Sea, so divers can progress from Open Water all the way to Divemaster and beyond.

A dive where divers let the current carry them along the reef rather than swimming against it. The boat picks up divers wherever they surface (usually marked by an SMB).

Common at Red Sea sites with predictable currents — Police Station, Elphinstone, Ras Mohammed, Jackson Reef. Drift dives are usually the most relaxing way to cover a lot of reef without burning air against the flow.

See also: SMB

An inflatable bright-coloured tube divers deploy from depth before surfacing, signalling their position to the boat. The "delayed" in the name means it's released during the dive (typically at the safety stop), unlike a regular SMB which marks position at the surface.

Required equipment for current-prone Red Sea sites like Elphinstone, the Brothers, and Carless Reef. Most operators provide one; bring your own if you have a preference.

See also: SMB

A CPR and first-aid course — primary and secondary care — that is separate from diving but required before the Rescue Diver course. The skills apply on land or at sea, to anyone, not just divers.

It is a short, practical programme and a sensible certificate for any traveller to hold; Aquarius offers it alongside the Rescue course.

A species found only in one geographic region. The Red Sea has unusually high endemism — about 10% of fish species and many corals are found nowhere else. The combination of warm water, high salinity, and 5-million-year isolation from the wider Indian Ocean drove this evolutionary divergence.

Examples of Red Sea endemics include the Red Sea bannerfish (Heniochus intermedius) and certain Chromodoris nudibranch species.

Balancing the pressure in your ears and sinuses against the rising water pressure as you descend, usually by gently pinching the nose and blowing (the Valsalva manoeuvre). Equalising early and often prevents ear pain and injury.

If you can’t equalise, stop, ascend slightly and try again — never force it. Colds and congestion make it harder, so it pays to dive healthy.

Arabic for an isolated coral pinnacle rising from the seabed. Many Red Sea dive sites are named with "Erg" prefix or include it — Erg Somaya, Sha'ab El Erg, Erg Abu Ramada. An erg is essentially a single underwater mountain of coral, often only 50-100m wide but rising from deep sand to near the surface.

Diving on a single breath without scuba equipment. Distinct from snorkelling (which stays at the surface) and scuba (which uses compressed air). Modern competitive freediving has divers reaching 100+ metres on a single breath.

Dahab is one of the world's premier freediving destinations — the Blue Hole hosts world-class training and competitions. Recreational scuba divers will encounter freedivers training around the Blue Hole's edges.

Standard entry technique from a stable platform (boat deck, dock, shore ledge). Diver takes one large step off, holding mask and regulator firmly. The "giant" refers to the leg position — a long stride keeps the body upright and prevents the BCD from slipping over the head on impact.

Small transparent fish that form dense schools in shaded reef areas — typically inside wrecks, under overhangs, and in caves. Their transparency is camouflage; predators struggle to single out individuals from the shimmering mass.

The El Mina wreck and Umm Gamar's "Cathedral" are famous glassfish sites in the Red Sea.

Hurghada Environmental Protection & Conservation Association. A non-profit founded in 1992 that maintains the mooring buoy system across Egyptian Red Sea reefs (preventing anchor damage), runs coral nurseries, monitors dolphin populations at sites like Sha'ab El Erg, and coordinates dive operator environmental practices.

The dive permit fee divers pay in Hurghada is partly used to fund HEPCA's work.

A dive operator's directly accessible shore reef. Distinct from "boat dive" sites that require boat transport. Aquarius Sharm's house reef is at Ras Ghamila; our Makadi and Sahl Hasheesh bases also have walk-in house reefs.

House reef diving allows multiple dives per day, easy training environments, ideal night dives, and refreshers after long surface intervals. A good house reef is a major operator advantage.

The PADI course that trains experienced divers to become scuba instructors, run as an intensive programme and assessed at a separate Instructor Examination. A centre that runs IDCs holds the highest training rating: PADI 5-Star IDC.

Aquarius is a PADI 5-Star IDC centre — the same team that guides your first breath also trains career instructors.

A multi-day boat trip where divers sleep aboard. Used for offshore Red Sea destinations — Brothers, Daedalus, Elphinstone, St John's — that are too far for day trips. Typically 6-7 nights, 3-4 dives per day, all meals included.

Liveaboards are how serious divers reach the southern Red Sea's pelagic action. The boats range from budget ($800/week) to luxury ($3,000+/week).

Arabic for "anchorage" or "harbour." Used as a prefix in Red Sea place names — Marsa Alam, Marsa Bareika, Marsa Mubarak. A "marsa" is typically a sheltered bay good for boat anchoring and often (because of the protection) good for diving and snorkelling.

A core skill: removing water that seeps into your mask by exhaling through the nose while tilting the head back, without surfacing. Every diver learns it in their first course and practises it until it is automatic.

It is far easier than beginners fear; your instructor demonstrates it in shallow water long before you would ever need it on a reef.

The maximum time a diver can spend at a given depth before mandatory decompression stops are required for safe ascent. Recreational diving is planned to stay within NDL.

NDL shrinks rapidly with depth: roughly 56 minutes at 18m, 18 minutes at 30m, 9 minutes at 40m. Modern dive computers track NDL in real time and warn before it expires.

See also: DECO

The state in which a diver neither sinks nor floats but hovers effortlessly, achieved by balancing weights, breathing and the air in the BCD. Good buoyancy is the difference between a relaxed dive and a tiring one — and it protects fragile coral from fin damage.

It is the single most useful skill to refine; PADI even has a Peak Performance Buoyancy specialty devoted to it.

A reversible, alcohol-like impairment caused by breathing nitrogen under pressure, typically noticeable beyond about 30 m. It can dull judgement and coordination; the remedy is simply to ascend a few metres, where it clears at once.

It affects everyone differently and varies day to day, which is one reason recreational depth limits and careful gas planning matter.

Breathing gas with more oxygen than regular air (typically 32% or 36% O₂ versus air's 21%). Extends NDL and reduces nitrogen loading, allowing longer or repeat dives. Requires a separate certification (PADI Enriched Air Diver) — one of the easiest specialty certs.

For Red Sea diving with multiple dives per day, nitrox is genuinely useful. Most Egyptian operators offer it for a small daily surcharge.

Sea slug — colourful, often spectacular soft-bodied molluscs that have shed their shells in evolution. The Red Sea has 100+ species, ranging from 5mm pygmy seahorse-sized specimens to the 40cm Spanish Dancer (Hexabranchus sanguineus).

Nudibranchs are the macro photographer's holy grail. Many produce defensive toxins — never touch them.

See also: Nudibranch species guide

The backup regulator a diver carries for buddy emergency air-sharing. Always brightly coloured (usually yellow) and longer-hosed than the primary so it can reach a buddy who's out of air. Confusingly named after the cephalopod, supposedly because of the multiple hoses extending from a typical reg setup.

The PADI entry-level certification. Allows diving to 18 metres anywhere in the world without instructor supervision. Course typically takes 3-4 days and includes pool training, theory, and four open water dives.

For most Red Sea visitors, OWD is the right starting cert. AOW upgrades make sense after 10-15 dives if you're returning regularly.

See also: AOW, PADI Open Water guide

Professional Association of Diving Instructors. The world's largest dive training organisation, headquartered in California. PADI certifications are recognised globally and required by most Red Sea operators (or equivalent — SSI, BSAC, CMAS).

Aquarius is a PADI 5-Star Resort & IDC, the highest tier of PADI dive operator status, granted for service quality, professional standards, and equipment maintenance.

Open-ocean species, as opposed to reef-dwelling. Whale sharks, oceanic whitetip sharks, hammerheads, mantas, threshers, and tunas are all pelagic. They live in blue water and only visit reefs to feed or use cleaning stations.

"Pelagic action" is diver shorthand for sightings of these big animals, which is why offshore Red Sea sites (Brothers, Daedalus, Elphinstone) are prized over coastal reefs.

Entering the interior of a wreck, cave, or overhead environment. Recreational penetration is limited to areas with direct line-of-sight to open water. Anything beyond that requires technical training, redundant gas, and proper line-laying procedures.

The Thistlegorm cargo holds allow safe recreational penetration. The Salem Express has welded-shut areas due to remains still inside; respect those marks. The Blue Hole's Arch is technical penetration only.

A pressurised chamber that treats decompression sickness and serious lung-overexpansion injuries by recompressing the patient and delivering oxygen. Hurghada has a chamber serving the northern Red Sea.

Knowing one is nearby is reassuring, but the goal is never to need it — conservative profiles, slow ascents and safety stops keep diving very safe.

A short programme for certified divers who haven’t dived in a while, reviewing key skills and theory in shallow water before heading to the reef. Recommended if it has been a year or more since your last dive.

At Aquarius a refresher pairs naturally with a check dive on the house reef, so you regain confidence before a boat trip.

The breathing apparatus that delivers air from the tank to the diver. Has two stages — first stage attaches to the tank and reduces tank pressure; second stage is the mouthpiece that delivers air on demand. A well-serviced regulator is the single most important piece of safety equipment.

The PADI course that teaches you to prevent and manage diving emergencies — self-rescue, helping a tired or panicked diver, and accident management. It is widely called the most challenging and most rewarding recreational course.

It requires EFR first and is the last step before the professional ratings. Aquarius runs it across all four Red Sea bases.

Ferry design where vehicles drive on and off through bow or stern doors. Efficient for cargo and passengers but vulnerable in rough seas — when bow doors are open or fail, water floods the car deck rapidly. The Salem Express was a ro-ro ferry, and bow door failure played a role in her sinking.

A 3-minute stop at 5 metres on every dive deeper than 10 metres. Not strictly required by the dive tables but strongly recommended — it provides an extra margin of safety against decompression sickness by allowing more nitrogen to off-gas.

The shallow lip of a Blue Hole or sinkhole. The Dahab Blue Hole's saddle sits at 7m and connects the inside of the sinkhole with the outer reef wall. Recreational divers use the saddle as their exit/entry path between the open sea and the sinkhole.

Arabic for "reef." Used as a prefix in Red Sea dive site names — Sha'ab El Erg, Sha'ab Ali, Sha'ab Abu Nuhas, Sha'ab Sabrina. The plural is "Sha'ab" too in dive-site usage.

An inflatable bright-coloured tube deployed at the surface to mark a diver's position to the boat. Different from a DSMB, which is deployed during the dive at depth.

See also: DSMB

The rest time between two dives spent at the surface. Allows residual nitrogen to off-gas and reduces the risk of decompression sickness on subsequent dives. Most Red Sea day trips have a 60-90 minute surface interval between morning dives.

Diving beyond recreational limits — deeper than 40m, with planned decompression stops, in overhead environments (caves, deep wreck penetration), or using mixed gases (trimix, helium). Requires specialised training, multiple tanks, and rigorous gas planning.

Egyptian Red Sea offers world-class tech diving — deep wrecks, the Blue Hole's Arch, and offshore reefs that drop past 100m. Tec divers in Egypt typically need PADI Tec 60 or equivalent for most serious dives.

A sharp temperature transition between warmer surface water and cooler deeper water. Visible underwater as a shimmering, blurry boundary. In the Red Sea, thermoclines often form at 20-30m in summer and disappear in winter when the water column is more uniform.

Hammerhead sharks tend to gather at thermoclines, which is why they're often spotted at 25-30m at sites like Jackson Reef.

The horizontal balance of a diver in the water column. Good trim is flat and parallel to the bottom — feet, hips and shoulders all at the same depth. Poor trim (feet down, head up) drags fins through silt and burns extra air. Considered the marker of a skilled diver.

The horizontal distance at which a diver can see clearly underwater, measured in metres. Red Sea visibility is typically 20-40m, with peaks above 40m at offshore sites in autumn. Reduced visibility is caused by plankton blooms, sediment from current/storms, or thermocline-induced haze.

"What's the vis?" is the universal first question on any dive boat.

A dive along a near-vertical reef face that falls away into deep water. You glide beside the wall watching soft corals, gorgonians and pelagics out in the blue, controlling depth by buoyancy rather than following a slope.

The Red Sea has world-class walls — Ras Mohammed’s Shark and Yolanda Reef and the Tiran drop-offs among them.

The lead a diver wears — on a belt or in BCD pockets — to offset the natural buoyancy of the body, wetsuit and a near-empty tank, so you can descend and hold a safety stop. Correct weighting underpins good buoyancy and air use.

Centres provide weights; tell your guide your usual amount and expect a little fine-tuning on the check dive as wetsuit thickness changes.

Diving on a sunken vessel. Egyptian Red Sea has many world-class wrecks — Thistlegorm (cargo ship, WWII), Salem Express (ferry, 1991), El Mina (warship, 1970), the seven wrecks of Sha'ab Abu Nuhas, Numidia and Aida at the Brothers Islands, Dunraven, and many more.

Wreck diving is fascinating but unforgiving — silt-out, entanglement in cables, sharp metal edges, and disorientation are real risks even on familiar wrecks. Always dive with a knowledgeable guide.