In the HTFA games prior to HTFA 5, none of the playable characters can swim, so when they're placed underwater, they simply fall like normal, but they're affected less by gravity down there when it comes to jumping, allowing them to reach higher heights whenever they do so (except in HTFA 4). Because of this, any other body of water without safer land underneath is pretty much a bottomless pit in those games. In HTFA 2, there's one level that is completely underwater that requires swimming. For that, you get to ride either TwinBee (for male characters) or WinBee (for female characters). In HTFA 4, however, the playable characters can somewhat "swim" by having them jump "mid-air" underwater.
Starting from HTFA 5, all playable characters can swim properly underwater. One character even stands out for being able to handle very well underwater, having faster swimming speed.