Dominic Quesada (played by Gilbert Roland) is a protagonist in Underwater!.
A mercenary hired to join Johnny's group in searching for and recovering sunken treasure in the Caribbean waters. Gloria (played by Lori Nelson) appears in Underwater!.
The boat owner who provides the vessel for the group's treasure hunting activities and accompanies them on their dives and recovery operations. Father Cannon (played by Robert Keith) appears in Underwater!.
A priest traveling with the treasure hunting party, supporting the crew as they search for the wreck and plan to recover the 17th-century ship.