Paul McCartney -- Swim Baby Swim

Paul McCartney did what most celebs do around the holidays -- migrate to a warmer climate and hit up the beach!

McCartney and his wife Nancy Shevell went for a swim Friday while vacationing in St. Barts ... and the 71-year old singer was awfully spry for his age while diving into the water.

Celebs have been peacing out to exotic places around Christmas and New Years for decades ... with St. Barts, Mexico and Hawaii being the choice destinations.