The moment at which you can no longer keep your finger in the water for more than a second, it should be about 115 degrees F. (hot tubs are often set at 105F or less to prevent injury, and 110F is intolerable).
A hot dry surface on the other hand can be tolerated by touch for more than a second up to 130F. Water causes an increase in the transfer of heat by fluid thermodynamics.
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