I've had several "cat sized" droppings left around my pool. Luckily they've never been IN the pool. I was convinced the nieghbors little dog was getting in through a hole in the fence (that I could never find.) But then one night I saw a cat sized possum running along the top rail of my fence. Now I see him virtually every night I swim after dark. He hisses at me when I throw empty bleach bottles at him.

As I was writing this it occured to me that it may very well be a cat. You can buy a kit that will train a cat to use a toilet;

http://www.citikitty.com/

Heck, the cats in the pictures would have no trouble making a little deposit in the pool.