The Seas Beneath (1931)

In the final days of WWI, Commander Robert Kingsley leads a U.S. mystery ship, disguised as a merchant vessel but secretly armed to destroy German submarines. Towing a powerless submarine, the crew sails toward Gibraltar to confront the U-172, commanded by Baron von Steuben. During a stop in the Canary Islands, Kingsley meets Anna Marie, unaware she is von Steuben's sister. Amid espionage and betrayal, tensions escalate.