What is the effect of returning nuclear reactor cooling water back into bodies of water?

The effect would be dead and contaminated fish.

This is because the waste waters from the reactors have radioactive elements in them. When exposed to fish, the fish become contaminated and they die to radiation. If these fishes are consumed by organisms up the food chain, they too become contaminated, worse off due to bioaccumulation.

acts as a barrier disrupting fish migration patterns

warm water is harmful to fish because of its lower oxygen concentration of water