All mammals can swim.
I remember learning this in grade school as part of our introduction to the natural world. In fact, as I've learned since, there are very few animals that can't swim in one form or another. Many don't, or at least not often, but they are able should the need arise. Elephants, cows, snakes, tiny insects, kangaroos, baboons, gazelle, even the unassuming koala. Yet, I'm always surprised by how many people, both children and adults tell me that they can't swim.