River does, per millicanon; we've talked about family vacations to a bioluminescent lagoon, and stuff. I don't think she's a particularly strong swimmer, over distance or in really rough water or whatever, but she can stay afloat happily under normal conditions.

Regan, ditto, but she enjoys swimming laps more, or did when she had more free time for it. So she's a bit stronger.

Trowa can swim. Uh, I'm not sure he entirely understands the point of swimming recreationally in a "splashing around in a lake/pool/etc" sense, but understands "if I am dropped in the water or have cause to cross water without a vehicle, I should be able to cover however much distance I need as safely and efficiently as possible." He probably learned in lakes and oceans more than pools, given his childhood. Cathy probably drags him swimming sometimes and tries valiantly (and futilely) to get him to wrap his brain around the fun part of a swimming pool or beach.

Clare... I'm not sure. She has fights in lakes and bathes there and stuff, but they're all in shallow water. I'm not sure Clare can actually swim so much as leap off the bottom and out of even deep water.

Thor can swim, and has quite an excellent concept of fun! And is pretty strong and tireless, which is good since if he were dropped into water it'd probably be in full armor and with a hammer. (He can make the armor go away magically and drop the hammer for later retrieval, though. Sorry, any sea life swimming underneath!) Going to the beach or whatever would not be his first thought for fun, but he'd happily participate if someone else suggested it.