Vince Vaughn just couldn't keep his head above water yesterday -- finding it nearly impossible to keep his balance on a Jet Ski in the rough sea.
The plumping actor was filmed in the waters off Malibu, just down the beach from ex Jennifer Aniston's house ... where he attempted to thrust himself onto a wave rider not once, but twice -- knocking it over and slipping off each time. Vaughn overboard!
Third time was the charm for slippery Vinnie, and he was eventually ferried from the beach to a waiting boat, as a crowd on the sand cheered his successful mounting. And who wouldn't?