Have you ever noticed that you feel perfectly comfortable walking around in 20° Celsius/68° Fahrenheit but that water of the same temperature feels quite cold? It is because water is a much better conductor of heat than air. When diving, the water that comes in contact with your body warms up, expands, and quickly carries the heat away from you. As a result, you feel cold in no time. In effect, your body will lose heat (through thermal radiation and heat conduction) about 20 times faster in water than in air, thus emphasizing the importance of good thermal protection for a wetsuit, semi-dry suit, or drysuit.
Another reason for greater heat loss when diving is breathing on open circuit. Breathing on open circuit generally accounts for about a quarter of your body’s heat loss because your body has to heat inhaled cold compressed air for the gas exchange in your lungs to happen efficiently. After warming it up, warm air is exhaled, and the heat is lost. The inhaled gas from a regulator can be as much as 15° Celsius to 20° Celsius/59° Fahrenheit to 68° Fahrenheit colder than the surrounding water temperature. So, a diver cools from the inside out on every open circuit exhalation.
Temperature is important to consider when immersed. Even in warm water, a small drop in body temperature can be serious. Divers wear exposure suits to reduce heat loss. Diving suits are either sealed dry suits that do not allow any water in or wet suits where some water gets in.
Unfortunately, underwater physics throws divers another problem: the increasing pressure compresses the insulating gas bubbles inside the exposure suit, making it less effective the deeper you go. That’s why there are recommendations as to what type and thickness of wetsuit you should wear in different water temperatures. Consult the dive center in the area you are planning to dive regarding what type of protection would be best for you.