The coal in a coal fired power station never comes into contact with the process water. The condensate that comes out of the steam turbine is cleaner than the water that comes out of your tap, typically only a few μS conductivity. The cooling water is usually just river water, but a bit warmer and cleaner because it's been through a few filters and used for cooling. Some of the water is used for flushing the ash away to settling ponds, however, which you definitely wouldn't want to swim in. In a gas fired power station, there's obviously no ash, just cooling water and condensate.