Blue Island (1982)

Billy and Bonnie, two young people on the same flight to Sydney, are the only survivors of the plane crash. They land on an apparently deserted island, where they adapt to live; then they meet Shanghai, a primitive man who initially shows hostility, jealous of their relationship. Young people manage to get him away from the island, sending him on a raft in the middle of the ocean. Then, when both are stung by a poisonous mollusk, Shanghai returns to the island to save their lives, managing to have them embark on a ship to civilization.