Divers need to be more concerned about heat before and after a dive than during a dive. Even in warm water the body gets cool, so than the primary concern is with heat as a result of the above water activities.
When the body temperature rises, weather from a rise due to the exposure suit, or to the fact that the sun is out, or a combination of those, the body goes through several cooling mechanisms to protect the core temperature.
Initially, skin capillaries dilate, allowing heat from the blood to radiate to the skin. Perspiration follows, which cool the skin. If the core temperature remains high, the pulse races, to help circulate more blood to help cool the body. These responses continue until the core temperature returns to normal.
A diver's physiology also reacts the same to a temp drop to protect the core temperature. Water conducts heat more than 20 times faster than in air and requires much more heat to raise its temperature. This means temperature that seems just right in the air, may be considered cold underwater.
With out the proper exposure protection, the average diver will be dangerously cold in half an hour in 40 deg water. For the majority of divers, water must be warmer than 80 deg to dive with out a suit.
By insulating the diver, the suit will extend dive times by slowing the cooling process, and making the body efforts more effective.
Divers with more fat tissue tends to loose heat more slowly than those that are thinner.
As the body cools, temperature centers in the hands, feet and head trigger heat conserving response that begin a change in circulation. Blood flow to the extremities slows through vasoconstriction to reduce heat loss from the blood through the skin. If vasoconstriction does not sufficiently maintain the core temp then the body reacts by triggering shivering. Shivering signals a losing battle with the cold. Once this starts, you need to get out of the water and begin the warming process.
A diver extends the bodys ability to maintain the core temperature through proper use of exposure suits and diving techniques. The head loses heat because the body can not reduce blood flow to the brain. Therefore a wet suit hood makes a huge difference in reducing heat loss.
Now lets talk about wet suit compression. As you dive, the suit which has large air cells begin to shrink. Meaning that the deeper you go the less protection you have to the cold. And if the suit is old than the protection is lost to a greater extent.