If you haven't read Lynne Cox, check out:
http://www.lynnecox.org/booktour.htm
http://www.lynnecox.org/Lecture.htm
She has to be an exception among us, but as an example, she swam 30 minutes in 31 degree water in Prudhoe Bay. She swam the Bering Strait.
From my own limited experience, I can't...