Up Periscope (1959)

"The man they called the 'human torpedo' - the secret underwater marauder who hit like no man hit before!"

Lieutenant Braden discovers that Sally, the woman he's been falling in love with, has actually been checking out his qualifications to be a U.S. Navy frogman. He must put his personal life behind him after being assigned to be smuggled into a Japanese-held island via submarine to photograph radio codes.