Sudden immersion in cold water can induce rapid, uncontrolled breathing, cardiac arrest and other physical body conditions, which can lead to drowning. Always wearing a PFD will help survival in rapid immersion situations.
In other situations when entry into cold water is necessary:
Wear a PFD.
Button all clothing.
Cover your head if possible and enter the water slowly.
Keep your head out of the water if at all possible.
Assume the Heat Escape Lessening Posture (HELP) position as taught within a Coast Guard-taught safety course.
Information about HELP is available online here
Immersion in water speeds the loss of body heat and can lead to hypothermia, the abnormal lowering of internal body temperature. If a boat capsizes, it will likely float on or just below the surface.
To reduce the effects of hypothermia, get in or on the boat. Try to get as much of your body out of the water as possible. If you can't get in the boat, a PFD will enable you to keep your head out of the water. This is very important because about 50 percent of body heat loss is from the head.
It may be possible to revive a drowning victim who has been under water for some time and shows no sign of life. Cases document instances where victims have been resuscitated after extended periods. Start CPR immediately and get the victim to a hospital as quickly as possible.