picture from here
I was all set to dig up fascinating and weird truths about this most graceful of sea slugs, but the internet offers no such treasures. They can swim, and they're big for sea slugs, which is a succinct summary of what the internet hivemind knows.
I'm forming the opinion that they get away with this by being gorgeous and entirely hypnotic. Who needs to be able to hold a conversation when you do this all day?
(Just ignore the dumbtard diver poking the poor thing, and the further dumbtard diver dancing, and the random apperance of a lion fish. Actual spanish dancing is at the 1 minute mark - it's the best footage I found on YouTube.)
ETA: I stumbled across this page in my GISing. It's a competition gallery of underwater photography that is well worth eyeballing. See?