Private Arrangement (1970)

Office workers Gloria, Emily, Linda and Beverly join sailors Terry, Bob and the skipper for a weekend cruise on the Playpen. Bob and Linda and Terry and Beverly pair off while Gloria divides her time between Emily and the skipper. The boat anchors at a cove where the voyagers enjoy a lazy day. That evening, everyone takes off their clothes and engages in group sex. Freddy, the skipper's deaf-mute helper, tries to make love to Emily. The next morning Freddy is found beside Emily's dead body. The police are informed and Freddy is taken into custody.