Forbidden Island (1959)

A group of skin divers, led by Dave Courtney, are hired by Edward Godfrey to locate a priceless emerald buried in a sunken ship in the South Pacific. Along for the trip is Joanne, forced by Godfrey to pose as his wife. Diver Dean Pike finds evidence underwater that Godfrey had murdered his real wife. Godfrey damages Pike's diving equipment and Pike dies before telling what he has found. Courteny has been suspicious of Godfrey so the latter tries to frame him for Pike's death.