As a matter of fact I have gone swimming in a pond of wastewater that was not bone-chilling cold because the water was refreshed by the cooling tower of a nearby nuclear power plant. I've also taken a dip in a pond that was heated by a nearby AM radio transmitter's water cooling system, back when water-cooled vacuum tubes were commonly used for broadcasting. I've also gone into a swimming pool that had a non-functioning heater, and nearly died from hypothermia. One guess which was the worst hazard to life...