Chuck Liddell went for a swim in San Diego over the weekend ... though he didn't do it voluntarily, instead slipping off a boat into the bay!
TMZ has obtained video of Chuck hanging around on a boat, talking expressively with a couple dudes and seemingly leaning forward to show them a move with his fist balled.
Unfortunately, seems like CL didn't quite have his sea legs underneath him 'cause he started to fall backward -- and Chuck's companions weren't looking toward him, so they weren't able to stop his descent.