It sounds like you have an above ground pool? If this is the case the best thing to do is to cover the pool. Also, rodents can swim, but when they get into the pool they can't get back out, eventually becoming tired, and then their demise. You could also put a ladder in the pool or devise something that would allow the rodents to escape.
We had a tree rat problem, and in the winter we covered the pool, in the summer we put a couple of heavy ropes in the pool so that they would have something to "climb" out on. We placed 4 ropes around the pool and anchored each rope to the lip on the side of the pool with a rock or brick. A couple still drowned, but most got back out.