Reflex, or Inhalation Response, is an automatic physiological
reaction resulting from sudden immersion into cold water. When sudden
immersion takes place, the individual involuntarily gasps, which is a
reflexive sucking in of air in an attempt to rapidly increase oxygen
intake into the lungs. This increases the body’s metabolic rate,
building internal warmth in response to the cold. When a rescuer
jumps into cold water with his face unprotected, the sudden urge to
breathe deeply in response to the torso reflex, may cause the rescuer
to inhale with the mouth underwater. If so, the rescuer inhales water
rather than air into the lungs.

prevent Torso Reflex, a rescuer should always cover his nose and
mouth with his hands as he falls into the
water. The rescuer should also attempt to land on his back, which
also reduces facial contact during sudden immersion into the water.
The action of covering the nose and mouth with the hands should be
taught to all rescue personnel subject to suddenly
entering cold water. This procedure should be instinctive and should
be maintained until the rescuer regains a horizontal position on the
surface with his face out of the water. Maybe the last thing to do
when falling over in cold water in your boat should be to grab your
mouth and nose?

a posting to the Chesapeake Paddlers Association email list.