Wetsuits keep you warm by trapping a thin layer of water between the suit and your skin. Your body heat then heats this water. If the suit fits properly, that water will remain in your suit during your dive and you'll remain warm. However, if too much water flows into the suit your body will continue to heat to the water in the suit. This can result in chilling.

For this reason, it's important to select a suit that fits properly. The fit should be snug enough to trap a thin layer of water between your skin and the suit. But comfort is also crucial. You must have freedom of movement to swim and manipulate your gear. Additionally, a suit that's too tight will make breathing difficult.