The Incredible Mr. Limpet (1964)

Aquatic life fanatic Henry Limpet (Don Knotts) falls into the sea and transforms into a fish, quickly becoming pals with a crab and developing a romantic bond with a fetching female fish. As a human, Henry was passed over for naval service. But now that he's a fish, he seeks out his friend Lt. George Stickel (Jack Weston). Finding him on a small boat in the Atlantic, Henry explains that he can alert sailors when enemy ships are close by, and soon he's helping America win World War II.