In an ideal world, it would be wonderful to be neither hot nor cold underwater. To achieve this, wear exposure protection suitable to the conditions you’ll be diving in. If you don’t wear appropriate exposure protection, you could suffer from overheating (hyperthermia) and/or cold (hypothermia). Just 2° Celsius above or below the standard body temperature will lead to either of these conditions.
Water will conduct heat away from your body 20 times faster than air. So, what feels like a mild temperature out of the water can be an effective cold temperature in the water. Further, even with exposure protection, the length of submersion will eventually cool you down to the extent it becomes uncomfortable. As soon as this happens, get out the water and warm up!
Overheating can also be a problem before diving. Your exposure suit should be put on after getting your dive gear ready, just before the actual dive, and taken off right after exiting the water because a suit limits the normal emission of heat in the air. Remember, if you are not feeling comfortable for the environmental conditions that you are in, notify your buddy/divemaster/instructor, and take appropriate action to either cool down or, in the case of cold, warm up.