It was a star-studded afternoon on the water off the coast of St. Barts today! Matt and Luciana Damon popped up on a mega yacht with Chris Hemsworth and his wife, Elsa Pataky, and other friends. They spent the day lounging on the huge watercraft, which featured a helicopter and helipad, and taking turns riding down an inflatable slide. Matt went shirtless while Chris also showed off his hot Avengers physique sans shirt. Luciana donned a black bikini when she wasn't covered up in a white maxi dress, and Elsa wore a skimpy dark two-piece as well. It was just announced that Elsa and Chris are expecting their first child after getting married in 2010, and Elsa's growing baby bump was on display during the sunny vacation.