What animals have four legs and can't swim?

The usual answer is that camels cannot swim, but they can. There is a question whether some apes, such as gorillas, may be too top heavy to swim, but this is not proven either way. In any event, their bodies are not that much denser than humans in water. The armadillo is another mammal where at least one species *can* float and swim, but some other species may be unable to.