Live non-venomous snakes of all kinds, usually alive and swimming, usually juveniles. Also, copperheads, a few corals, drowned in skimmer, and two summers ago a juvenile rattlesnake, drowned in skimmer. For some reason the venomous snakes seem to drown more quickly than the non.
Last summer, when pool was drained for acid washing, an adult emerald green, with yellow belly, non-venomous, snake, 36", got into pool and baked in the 100+ morning temp/sun. It was noon before I found it at the drain where there was a bit of water. I thought I knew all of the snake types out here, and see at least one live snake every day. I had never seen this kind of snake before. I tried to identify it but I nothing I've found has helped me put a name to it.
We've never had any birds drown although we feed hundreds. No squirrels or other larger critters. Occasional rats and mice that I rescue if they haven't drowned, a dead mole and a couple of dead, rotten, rats one winter that I used a leaf catcher cover, frogs and toads too. I check the skimmer every morning and rescue anything living.
One winter, when we had a leak, pool was lowered and no pump running. Next spring we had thousands of tadpoles and left them to mature, with hundreds of frogs, before repairing pool.
BTW... every year we have at least on or two squirrels with damaged tails. Last two with absent tails we named Nada and Stubby. Last year we had one with broken tail, but still attached. In the fall she fell from a tree and met her demise.
I dont think I would want these critters swimming with me. I am very happy we have a pool enclosure :-D :-D 
Our explosion of frogs, when the pool became "frog pond", made for a HUGE explosion in Copperheads, too. We just have to be really careful, lots of lighting, flashlights, headlights, and no barefoot except on decks, and bring the younger dogs in at dusk to avoid so many snake bites. After so much time you get used to taking precautions for all critters, warm and cold "blooded".
