I learned to sail at Bradwell in Essex.  We used to do our capsize drills in the water coming out of the nuclear reactor there (not primary coolent, but the water used in electricity generation I think).  It was much warmer than the rest of the Blackwater Esturary.  We used to joke about glowing in the dark at night - although that never happened. I don't think people can do that now as the plant has long since shut down.