The Incredible Mr. Limpet (1964)

Meek and mild mannered bookkeeper Henry Limpet has few passions in life. It's mid-1941 and he would love to join the Navy but has been rated 4F. His friend George Stickle is in the Navy and lays it on pretty thick. If Henry could have one thing it would be to become a fish. While on a visit to Coney Island, Henry falls into the water and miraculously gets his wish. Now a fish, he makes friends, Ladyfish and Crusty the hermit crab and loves his new life. He also uses his abilities to help the US Navy locate and sink Nazi U-Boats, forcing the Germans to create a new weapon to deal with the Allies secret weapon. When years later the Navy finds intelligent activity among dolphins, they may also know who is teaching them.