Justin Bieber -- Throwin' In the Towel

Justin Bieber did one of the many things he does well ... strip down to his underwear, but this time it was a problem.

Justin was catching some rays in Laguna Beach Thursday when he shed his clothes and dove in the Pacific with just his Calvins.

Problem is ... when he hit the sand 10 minutes later he realized he didn't have a towel, so a random kid obliged.

Speaking of random ... Justin was there with Corey Gamble, Kris Jenner's BF.