Oh yeah.
As soon as we found the first one, we looked it up online and apparently it's kind of "the best kept secret" of swimming pools. No one mentions it, but it's apparently not uncommon at all for squirrels to end up dead in there. They can fall in, but they can't get out.
Dogs, too. Cats. Frogs.
They even make ramps you can install to help animals out. ha.