Think about this for a moment. Most animals live out in the wild - where do they drink from (puddles, ponds, streams, rivers, garbage containers, etc....) A pool is probably one of the cleaner sources of water for an animal. If you leave a water bowl outside for the dog it probably has all sorts of bacteria growing in it that you cannot see. As long as you don't put strange chemicals in the pool the dog should be just fine drinking from it.

Heck, we all swallow pool water when we go swimming and it does not hurt us. I many times will just taste the pool water (rather than hauling out the test kit) to determine my chlorine needs.