Whenever two substances are in contact, heat is transferred from the warmer substance to the cooler substance. This is called conduction, and is the reason we get hot or cold depending on weather conditions.

Because water is denser than air, heat is conducted from our skin about 25 times faster in water. This is why 55°F air is comfortable, but water of the same temperature can be bone chilling.

Conduction of heat continues until the temperatures of both substances equalize. Unfortunately, we experience hypothermia when our body temperature drops just a couple degrees. Shivering is a warning sign that your body temperature is dropping, so abort your dive immediately if you start shivering.