We have a poll on hypothermia, but I thought I'd bring this to people's attention. Chances are with cold water immersion, you won't live long enough for hypothermia to set in. Here's one article, google cold water immersion and drowning, and you will have lots of reading.
The Shocking Reality
By Jeffrey Pollinger, Public Affairs, Coast Guard District 13


Think you're a good swimmer? It may not matter if you suddenly and unexpectedly end up in the frigid coastal and inland waters of the Pacific Northwest. Every year, dozens of swimmers and boaters drown in lakes, rivers and coastal waters in the Pacific Northwest. But it may surprise you that many victims don't die as a result of poor swimming skills or the effects of hypothermia, but from the immediate effects of cold water immersion, or cold water shock.


Unlike hypothermia, the effects of cold water immersion can lead to death in just a few minutes and in some cases, instantly.


Sudden entry into the water can cause cardiac arrest, even for people in good health. The shock of the cold water can also cause an involuntary gasp reflex that can cause victims to inhale water and drown. After just a few minutes, the ability to swim or tread water is impaired as the victim loses muscular coordination. All of this can occur in water as warm as 69 degrees.


"Sudden cold-water immersion is a phenomenon that is becoming more recognized as a cause of death as compared to hypothermia," said Dan Shipman, recreational boating safety specialist with the Coast Guard's Thirteenth District office in Seattle.


True hypothermia usually doesn't normally set in until at least 30 minutes after being in the water, depending on body size and type, insulation of clothing and other factors. Even then, victims can survive for hours before losing consciousness and drowning.