Intermediate 8 minutes
Hyperthermia: the risk that happens before you enter the water
Heat stroke on deck, dehydration, and why the tropics cool you down badly too
In tropical diving, the hottest moment of the day doesn't happen under the sun: it happens inside a sealed wetsuit, standing on a shadeless deck, waiting for the briefing fully geared up without a hint of breeze.
In this article you'll see why divers overheat outside the water and not inside it, how heat exhaustion and heat stroke differ, why dehydration connects heat to decompression risk, and what simple routine prevents all of it.
It's the most common thermal problem in warm destinations and the easiest to prevent, while also being one of the biggest contributors (through dehydration) to a risk that is genuinely serious. This article is educational material and does not replace specific first-aid training or medical judgment.
Why the problem is on deck
The body sheds heat mainly through sweat evaporation, and that pathway needs two things: skin exposed to air, and air that can carry the moisture away. A sealed wetsuit removes the first, and a shadeless, breezeless deck with high humidity removes much of the second.
The result is that a geared-up diver standing around is, thermally speaking, someone exercising with an insulating layer on. If they're also carrying the tank, walking to the entry point, or getting suited up under the sun, heat production adds on top of the insulation.
Once in the water, the problem nearly always disappears: even in tropical water at twenty-eight or twenty-nine degrees, water conducts heat far better than air does, and the balance flips immediately. That's why diving-related hyperthermia is a surface phenomenon, and in someone who is well it usually eases once they enter the water; someone who already feels unwell shouldn't be sent into the water but cooled down first.
The special case is drysuit diving in summer, where the insulation is even greater, and pre-dive overheating is a common complaint among people who dive cold water during the warm months.
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