I don't see how that follows - even with lacking care to mix exhaust hot water with cold to make it warm. That would at worst cause localized fish kills and algae blooms as opposed to lasting damage.

Nuclear plants are usually located with abundant water for cooling and contamination is more of a concern than evaporation.

Read someone mention that some reactors in France are limited by river temperatures in the summer. Or else they'll start cooking fish.

Me I think we're in the harm reduction phase of industrial civilization. Nothing we do isn't going to have some negative impact.