Arctic Water Survival and Anticipatory Thermogenesis :

The Arctic water temperature is below zero. Survival time in water is 5 to 10 minutes, if you don't die of cold shock in 47 seconds.  Body heat is lost 25 times faster in cold water than in cold air.

Cold shock is the physiological response to sudden cold and may cause a heart attack. Humans' standard operating body temperature is 37.5 °C.

If body temperature drops by even a small amount, hypothermia sets in.  Some people can use what is called anticipatory thermogenesis to raise their body temperature by up to 1.5 °C.

But if you dont here is what will happen as your body temperature falls :

At 35 °C you will be unable to write your own name.

At 33 °C you become irrational, throwing away survival gear.

By 32 °C you will collapse.

At 30 °C loss of consciousness with probable cardiac arrest.

By 28 °C irregular heartbeat, by 20 °C your heart stops. Your dead.