Although it might seem contrary to usual heroic fantasy, most
DMs maintain realistic limitations on the physical prowess of their
players’ characters. A logical appraisal of normal human (and
demi-human) capacities acknowledges that the effects of
extreme cold can be harmful to characters, even when not delivered
in the form of magical damage. Thus, the restrictions on a
character’s ability to resist cold are as follows:

Although the underworld is not a common setting for glacial
and arctic adventuring, the damp chill of the air is decidedly
unhealthy for most creatures. The streams, rivers, and lakes of
the Underdark have the capacity to numb a character who suffers
prolonged exposure to their bone-chilling cold waters.
A character becomes subject to hypothermia (a disastrous lowering
of body temperature) after one turn of swimming in a typical
underground body of water. This assumes that no subterranean
heat source is warming the water. The risks of hypothermia are
reduced in shallow water. If a character is moving +hrough water
between knee and waist deep, hypothermia effects do not begin
for one hour; if the water is shallower than knee deep, hypothermia
is not a threat. After hypothermia sets in, the player must
make a Constitution Check. If the check fails, the character loses
ld6 hit points. The check must be repeated every turn, with all hit
point losses accumulating until the character reaches some
external heat source that can warm his chilled body. Hit point loss
continues even after the character has gotten out of the water, if
no heat source is found.

If a means of warming up, such as a fire or the close presence of
other warm-blooded creatures, is located, the character regains 1 d6
hit points per turn until the losses have been fully recovered.
Creatures that are especially adapted for cold environments or
who carry magical protections such as a ring of warmth are not
susceptible to the effects of this rule.

A character who takes care to thoroughly coat himself with a
layer of grease or oil before entering the water can double the
amount of time before making a Constitution Check, and can
double the interval between checks as well.