The Terror Beneath the Sea

Kaitei daisensô is a movie starring Shin'ichi Chiba, Peggy Neal, and Franz Gruber. A group of surface dwellers comes upon an underwater city ruled by a mad scientist and his amphibious servants.
While covering a test of guided torpedoes, two reporters believe they see what appears to be a strange-looking swimming creature. They investigate the matter further and discover that there is a race of fish-men living under the sea. The fish-men capture the pair and keep them prisoner in their underwater city.