Seas Beneath (1931)

As World War I draws to a close, a U.S. "Mystery Ship" embarks on a mission to the Spanish coast, towing a submarine aimed at tracking down a particularly deadly U-boat targeting Allied vessels. Disguised as an innocuous schooner, the ship is secretly armed with a powerful gun designed for U-boat destruction. During a refueling stop in the Canary Islands, the crew encounters locals mingling with Germans, including the sister of the lurking U-boat commander, setting the stage for an impending confrontation at sea. Tensions rise as the battle looms on the horizon.