Interestingly, that's not because you'll get a thermal shock that will kill you. The body is surprisingly good at maintaining temperature for a while after being plunged in icy cold stuff. The problem is, it's so good at it, that it will block off the arteries to your legs and arms to stop them from leaking so much heat (better survive without limbs than die with them), and you'll drown because you can't swim. As far as hypothermia is concerned, you have about an hour before passing out due to hypothermia. But you have about 10 minutes if you're not wearing a life jacket.

http://mariovittone.com/2010/10/1-10-1/