Hypothermia

A dangerous drop in core body temperature; a constant risk for divers because water conducts heat away from the body about 25 times faster than air.

Hypothermia is a drop in core body temperature below about 35 C (95 F), and divers are exposed to it far more than most people realize because water conducts heat away from the body roughly 25 times faster than air. Even tropical water at 28 C is well below body temperature, and long or repeated dives slowly drain heat. The progression runs from feeling cold and shivering, through clumsy hands and sluggish thinking, to the point where shivering stops, which is a serious warning sign, not an improvement.

For divers the practical problem starts long before true hypothermia. Cold thickens the mind and fingers, increases air consumption, worsens narcosis, and is a suspected contributor to decompression sickness risk because chilled tissue off-gasses poorly. The defense is exposure protection matched to the water: a wetsuit of appropriate thickness, a hood, which matters greatly given how much heat the head sheds, gloves, or a drysuit for cold water. Thermoclines can drop the temperature sharply at depth, so dress for the coldest water on the profile, not the surface reading.

The rule most divers actually use: if you are shivering, the dive is over. End the dive, get dry, add layers, and take warm fluids. Between dives, get out of the wind, peel wet suits, cover your head, and warm up gradually: sitting in a soaked wetsuit through a breezy surface interval sheds more heat than the dive itself did.