The effects of cold water on the head and chest can render you unconscious instantly, although this outcome seems to be uncommon.
The momentum of an individual falling overboard usually drives the victim under the surface for several seconds. Exposure of the head and chest to cold water causes involuntary gasping (photo).
When the victim is not wearing a life jacket, the gasping may occur under water. In some cases, cold water is drawn into the lungs, in others, the muscles controlling airflow into the windpipe (trachea) close and stay closed. Either outcome prevents effective breathing when the victim surfaces. The victim, now unable to breath, is last seen struggling briefly at the surface before disappearing from view. Press reports tell us that the boaters drowned, leaving the impression that they just didn't know how to swim.

BEING ABLE TO SWIM IN THE WARM WATERS OF SUMMER HAS NOTHING TO DO WITH SURVIVAL IN COLD WATER.

Cold water immersion gasping reflex.

Survival tables suggest that a victim may survive for a relatively long time in cold water (40-50 degrees F, 1-3 hrs). These tables are only valid if the victim is wearing a life jacket. Without the flotation of the life jacket, the victim is forced to swim, which drastically accelerates heat loss, onset of paralysis, and drowning.