Skin Deep / Senza buccia (1979)

The plot focuses on a trio of young people; a young Casanova, his girlfriend and another friend. They're on a sailing trip, and it's turned upside down when they come across a Norwegian couple whose boat has broken down. The Norwegians are nudists, and after all five of them arrive back at a beach house; the young trio decides to join in by ditching their clothes, and creating sexual tension within the group.