Descend far enough almost anywhere in the world's oceans and the water turns cold. Below about 200 m (656 ft), the global ocean averages only about 4 °C (39 °F) 3. The Red Sea breaks that rule, and Hurghada sits beside the very part of it where its deep water is made. Below roughly 140-300 m, all the way to the basin floor more than 2,000 m down, the Red Sea is filled with an almost uniform body of water at about 21.5 °C (71 °F) and a salinity of about 40.5 1.
For technical divers, that one oceanographic fact touches almost everything: what to wear for long decompression, how a dive computer converts pressure into depth, how a rebreather scrubber behaves, and why the hardest part of a hot-weather tech day is often on the boat. This article explains why the Red Sea is warm all the way down, and what that does, and does not, change.
Why the deep Red Sea stays warm
The explanation is a wall under the water. The Red Sea is a long, deep, almost enclosed basin. Its only natural connection to the open ocean is a narrow strait at its far southern end, where the seabed rises to a shallow ridge only about 137 m below the surface 1.
Because that ridge is so shallow, the cold water that fills the deep ocean elsewhere cannot flow in. Below the ridge, the Red Sea is cut off from direct exchange with the open ocean 1, so its deep water has to be made locally.
That happens in the far north, the part of the Red Sea where Hurghada sits. Strong evaporation makes the surface water steadily saltier as it flows north, until it exceeds a salinity of 40 18. In winter, cooling makes this very salty water dense enough to sink. Dense water flowing out of the Gulf of Suez has long been seen as a major source, and recent work shows that open-sea sinking in the northern Red Sea during unusually cold winters can renew large parts of the deep water within about a decade 1.
The water that sinks is cold only by Red Sea standards: northern winter surface temperatures stay in the low 20s °C 24. The deep basin therefore sits at roughly the temperature of a northern Red Sea winter.
What the water column looks like through the year
"Warm all the way down" is accurate, but the column is not identical from top to bottom all year round.
A long-term analysis of six decades of temperature profiles found that a seasonal temperature cycle exists only above about 80-100 m in the Red Sea; below that, temperatures are stable 2. Surface temperatures in the northern Red Sea swing roughly 4 °C below the annual mean in February and roughly 4 °C above it in July-August 2. Around Hurghada, that works out to about 22 °C (72 °F) in late winter and spring and about 28 °C (82 °F) in August 4.
So there is a thermocline, and it is seasonal. In the north it is deepest in February, reaching about 80 m on average, and shallower in summer 2. In practice:
- Winter and early spring: the upper water is only slightly warmer than the deep water. The whole column, from the surface to well beyond technical diving depths, sits in a narrow band in the low 20s °C.
- Summer and autumn: the surface layer warms to the high 20s °C, and divers descending to typical technical depths pass through a temperature gradient into water a few degrees cooler, before reaching the stable deep water at about 21.5 °C 12.
Exact temperatures at any given site and date vary with local conditions, so treat these as orientation, not a forecast.
| Northern Red Sea | Typical open ocean | |
|---|---|---|
| Surface, late winter | ~22 °C (72 °F) 4 | varies widely |
| Surface, August | ~28 °C (82 °F) 4 | varies widely |
| Seasonal signal | upper ~80-100 m only 2 | varies |
| Deep water (below ~300 m) | ~21.5 °C (71 °F), salinity ~40.5 1 | ~4 °C (39 °F) average below 200 m 3 |
Salty water and your depth gauge
A dive computer does not measure depth. It measures pressure and converts it to a depth reading using an assumed water density 5. Common settings, using one manufacturer's published values, are fresh water at 1,000 kg/m³, the EN 13319 standard value at 1,020 kg/m³, and salt water at 1,030 kg/m³ 5. EN 13319 is the default on many computers and works out at almost exactly 1 bar for every 10 m 5.
Red Sea water is denser than the EN 13319 value because it is so salty. Calculated with the international seawater equation of state (TEOS-10) 6, water at 21.5 °C and salinity 40.5 has a density of about 1,028.5 kg/m³ at the surface, and summer surface water at 28 °C and salinity 40 about 1,026 kg/m³. Using about 1,028 kg/m³ as a working value:
- A computer set to EN 13319 shows depth = actual depth × 1,028 / 1,020, so it reads about 0.8 % deeper than you really are.
- A computer set to Salt (1,030 kg/m³) shows depth = actual depth × 1,028 / 1,030, so it reads about 0.2 % shallower.
| Actual depth | EN 13319 display (1,020 kg/m³) | "Salt" display (1,030 kg/m³) |
|---|---|---|
| 30 m (98 ft) | 30.2 m | 29.9 m |
| 60 m (197 ft) | 60.5 m | 59.9 m |
| 100 m (328 ft) | 100.8 m | 99.8 m |
The differences are small, well under 1 m even at 100 m, and the setting affects only the displayed depth. Decompression and oxygen calculations work from the pressure the sensor measures, so the salinity setting does not change the decompression the computer calculates. What it affects is consistency: team members with different settings see slightly different depths at the same stop. Because an EN 13319 reading is, in effect, "10 m per bar", it also matches the rule of thumb many divers use for gas limits. Know your setting, and agree on it as a team.
Thermal status and decompression
Most of the discussion of temperature and decompression goes back to a controlled study by navy diving researchers, published in 2007 by Gerth, Ruterbusch and Long 7. In the main test series, divers carried out controlled air decompression dives to 37 msw (120 fsw) while fully immersed in water held either warm (about 36 °C / 97 °F) or cool (about 27 °C / 80 °F), with the bottom and decompression phases controlled independently 8. Across 400 dives there were 21 cases of decompression sickness 8.
The result was clear in direction. Being warm on the bottom and cool during decompression gave a DCS rate of 22 %; being cool on the bottom and warm during decompression gave 1.3 %, even though that condition was extended to more than twice the bottom time 10. The effect of a roughly 9-10 °C warmer decompression was comparable to halving bottom time 8. The physiology is straightforward: warmth increases blood flow and therefore inert-gas uptake on the bottom, and increases elimination during the stops 11.
It is just as important to understand what the study does not say. The conditions were deliberately extreme, and the "cold" water, at about 27 °C, is warmer than a Red Sea winter. Researchers involved in diving medicine have cautioned that the results do not mean divers should chill themselves on the bottom, that the main series used a single dive profile (the lead author notes that dives to 46 msw / 150 fsw were also tested 9), and that most divers cannot control their temperature phase by phase 8. The lead author has also written that the direction of the effect almost certainly applies to recreational and technical divers, and that any active heating should be kept to the minimum on the bottom and increased only during decompression, without causing heat stress 9.
What this means in the Red Sea, with the caveat that no study has tested it there:
- The bottom-to-deco temperature difference is small. In winter, bottom and stop depths are within a degree or two of each other. In summer, the shallow stops can be several degrees warmer than the bottom, which leans in the favourable direction.
- Long deco is still a slow cooling. Water conducts heat 20-27 times faster than air, and hypothermia can develop slowly even in water at 29-33 °C with no protection 13. Neutral comfort for a swim-suited diver at rest is around 34-36 °C 8. Two or three hours of decompression in 22 °C water will cool an underprotected diver, which is exactly the "warm on the bottom, cold in the deco" pattern the research warns against.
- Heated vests deserve care. Following those findings, heating early in the dive increases gas uptake when it is already highest 10. Many divers therefore leave heating off, or low, until the ascent.
None of this is individual medical advice. Divers with specific health questions should talk to a dive physician or to DAN.
The heat problem is on the surface
In the Red Sea, the more immediate thermal risk is often above the water. Around Hurghada, average daily highs stay above about 33 °C (92 °F) from late May to late September, with August averaging about 36 °C (96 °F) 4. Technical divers spend long periods in that heat wearing thick exposure protection while assembling, carrying and checking twinsets, stages or a rebreather.
DAN notes that prolonged time on deck or on shore before entering the water, fully suited, can cause heat stress and exhaustion, with dizziness and nausea as common symptoms 12. Its advice is practical: avoid fully suiting up until just before you put on your buoyancy compensator and enter the water, limit direct sun while suited, stay hydrated, and remove gear and at least the top half of the suit after the dive 12.
Hydration matters, but in proportion. Dehydration can increase DCS risk 10, and pre-dive hydration has been shown to reduce circulating bubbles 17; DAN also notes that dive profile, thermal stress and exertion are more important risk factors 11. Heavy exertion near the end of or soon after a dive can promote bubble formation 11, so hauling heavy kit up a ladder straight after long decompression is worth planning around. A hot shower or hot tub right after diving can also promote bubble formation 10.
Exposure suits for long decompression
There is no single right answer for the suit; these are considerations, not prescriptions.
- Total immersion time drives the choice, not water temperature alone. A 22 °C dive of 60 minutes and one of 180 minutes are thermally very different dives.
- Neoprene compresses at depth. A wetsuit loses part of its thickness and insulation under pressure 14, so a suit that feels warm at the surface insulates less on the bottom of a deep dive. Its insulation recovers somewhat on the ascent, which helps during the stops.
- A drysuit with thin undergarments gives more control over the thermal balance across a long dive, at the cost of more heat on deck.
- Plan the surface as part of the suit choice. The warmer the suit, the more important shade, timing and cooling on the boat become 12.
Rebreathers in warm water
Warm water is good news for a closed-circuit rebreather. The reaction in the carbon dioxide absorbent produces heat and works more efficiently when the canister is warm. Cold water does the opposite: canister performance declines, and duration becomes more variable, as temperature falls 16.
Scrubber endurance figures under the rebreather standard EN 14143 are measured with the unit submerged in water at 4 °C 15. A diving physiologist writing for the rebreather industry association estimated that diving in 15 °C water may increase endurance by about 50 % compared with 4 °C, and diving in 30 °C water by a further 25-40 % 15.
The tested figure is a cold-water figure, so in a sea that is about 21.5 °C even at depth, the absorbent is working in its favourable range. That is a margin, not a reason to stretch the canister. Real duration also depends on workload, depth, the absorbent brand and how the canister was packed 1516. Follow your manufacturer's limits regardless of water temperature.
Key takeaways
- Below roughly 140-300 m, the Red Sea is filled with water at about 21.5 °C and salinity about 40.5, all the way to more than 2,000 m 1, compared with about 4 °C average below 200 m in the global ocean 3.
- The reason is a shallow ridge, about 137 m deep, at the Red Sea's southern entrance, which keeps cold deep ocean water out, while the deep water forms locally from warm, very salty winter water in the north, where Hurghada is 1.
- There is a seasonal thermocline in the top ~80-100 m: surface temperatures near Hurghada range from about 22 °C in late winter to about 28 °C in August 24.
- Because Red Sea water is dense, an EN 13319 computer reads about 0.8 % deeper than actual; the setting does not change the decompression calculation 56.
- Research favours being cool on the bottom and warm during decompression 789. Long deco in 22 °C water can still cool an underprotected diver.
- Heat stress on the boat, in full kit, is a real risk; follow DAN's suiting and cooling advice 12.
- Warm water favours CCR scrubber performance compared with the 4 °C test standard, but it is a margin, not an extension 15.