Waste water on power plants only go in and out of a heat exchanger called the Condenser, which turns the low temperature steam going out of the last turbines into liquid so that it can be pumped back into the boiler/reactor/heat exchanger then back into the turbines. You want only liquid water going thru the pumps, thus the need for the condenser. So this water never really touches the water going thru the turbines and should be safe to swim into, unless additives are used to prevent damage to the condenser. And the water inside the cycle isn't really dumped in enough quantities to make these pools, mostly you only get rid of some of it to purge impurities remaining from the treatment. Same goes for nuclear power plants, the water inside the reactor isn't the same water going thru the turbines and neither is the water going thru the condenser. They never touch each other.