swim speed

A creature that has a swim speed, such as a crocodile or a black dragon, doesn’t have to make Athletics checks to swim. It can simply move through water using its swim speed, doing so as part of any of its actions that involve it moving. For rules on swimming, see “Athletics”.

Published in Monster Manual 2, page(s) 219, Rules Compendium, page(s) 316, Monster Vault: Threats to the Nentir Vale, page(s) 125.