Heat loss in water is 25X faster than it is in air. So, while a 65 degree Farenheit day feels warm to most people, immersion in water at that temperature--sans wet suit--is life threatening. Water temperatures between 60F and 70F can cause a cold shock response. People have trouble breathing. Those who have not been properly acclimated can hyperventilate.
Most lap pools are kept between 77F and 82F. That's considered cool enough to keep someone swimming vigorously from becoming overheated. The Caribbean Sea is an average of 82F during September.
The coldest water ever survived in for a significant period was about 40F. An Icelandic fisherman was tossed into the sea when his boat capsized. The man swam for six hours in those water temperatures and was the only person aboard the vessel to make it home alive. The reason? He was obese. At 6'4 and 275lbs, his BMI was greater than thirty. Or, roughly, the same BMI as a seal.
--Source: The National Center For Cold Water Safety
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