What factor is critical for preventing hypothermia during cold water diving?

The critical factor for preventing hypothermia during cold water diving is the thermal protection provided by the dry suit. A dry suit is specifically designed to keep divers warm by creating an insulating layer of air that prevents the cold water from directly contacting the diver's body. It minimizes the loss of body heat, which is essential when diving in frigid conditions.

The dry suit also allows for the use of thermal undergarments, which can provide additional insulation. Together, these elements help maintain the diver’s core temperature, significantly reducing the risk of hypothermia. It is important to ensure that the dry suit fits properly and is maintained in good condition to maximize its effectiveness.