Last Woman on Earth (1960)

"They fought for the Ultimate Prize!"

Harold Gern, a shady businessman from New York, is spending a holiday in Puerto Rico with his attractive wife Evelyn. They are joined by Martin Joyce, Harold's lawyer, who has come to discuss the latest indictment. Harold invites him along on a boat trip during which all three try out some newly bought scuba diving equipment. When they resurface, they find out that the world has changed forever.