Why do fishes survive in cold water rather than hot water?

It's not the swimming, it's the food. Hot water, like in the tropics, is hard on plant life and plankton, two things fish eat. If you've ever been to the tropics, you will remember how clear the water is - that's because there are very few microorganisms living there. Cold water, on the other hand, is full of algae and plankton! Fish have plenty to eat, and many of them prefer to live in colder water because of that.