Warm Water Hypothermia
It's a common misconception that hypothermia is an exclusive concern of those who dive in cold water. So, if you think tropical divers are immune to the problem, think again. There is, in fact, a well-documented phenomenon known as warm-water or silent hypothermia.
Unless the water temperature is greater than 90 degrees Fahrenheit, you will lose heat. But unlike in cold water, heat loss in warm water is slow and gradual. This long, slow cooling of the body can substantially lower core temperature. The phenomenon has been documented in subjects engaging in diving for several days in temperatures as high as 81 degrees Fahrenheit.
While the mechanism is not yet fully explained, the theory goes like this: Under such circumstances, the skin temperature remains within the comfort range, while the core temperature slowly drops over time. The most common symptoms of undetected hypothermia are fatigue, loss of motivation and impaired mental ability, all of which can have serious implications. Some researchers even believe that this form of cooling may not stimulate shivering until the diver's core temperature has already dropped significantly. So, remember, unless water temperature is above 90 degrees Fahrenheit, the question isn't whether you're losing, heat; it's how much and how fast you're losing it.
How to Avoid Hypothermia
For starters, consult your local dive center for advice on exposure protection.The staff can help you select adequate exposure protection for the environment. Wear more exposure protection than you think you'll need. Probably the wisest investment a tropical water diver can make to avoid heat loss is to buy a hood.
Make sure your wet suit fits well to avoid water exchange and convective heat loss. Take care in considering undergarments for your drysuit.
Bring sufficient clothing for when you get out of the water. It should be warm and windproof. Again, the guideline is to bring more than you think you'll need.
In the air, a damp wet suit works like a refrigerator. So, if you'll be wearing your wet suit between dives, or on your ride back to the dock, bring a windproof jacket or overgarment to cover your exposure suit. This will prevent heat loss through rapid evaporation.
Get out of the water when you feel cold. Don't wait for shivering to occur as it could be delayed by a number of factors such as heavy exercise, alcohol consumption and even nitrogen narcosis. Avoid alcohol completely before diving and, if you're cold, even after. It may make you feel warm, but that's deceptive; it actually accelerates heat loss.
If you do begin to shiver, exit the water immediately and re-warm. While shivering helps the body rewarm, this works only on land. Shivering in the water only promotes further heat loss. You cannot rewarm in the water.