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.

Liddell made a big splash into the water, and the alarm bells sounded with the whole crew kicking into rescue mode to drag Chuck back on board.

One of the guys on the boat jumps off the back and swims toward Chuck helping him back up. He's got guys pulling on his arms while another supports his back as a little dog looks on.

It takes a while for his buddies to pull him back up onto the deck -- Chuck's a pretty big dude -- but ultimately they dragged him up out of the water.

Acting like the champ he is ... Chuck just kept on vibing into the evening, eyewitnesses say he had food delivered to the Lamborghini ship.

He also waved hello to a collection of fans -- though he didn't leave the ship for the rest of the night.