Hypothermia
Even if the water is 75 Deg F you can get hypo. it may take several hours, but it will happen. I believe it is needed to be 85-90 or above. Caribbean waters, basically.
If you think about it, your body core is 98.6. Anything less than that is going to pull the heat from your body and cold water does that much faster than air. This the reason a 75 degree day wont freeze you, but being up to your next in a 75 degree water body will, eventually.
Jason
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