Kaitei daisensô (1966)

Director: Hajime Sato
Cast: Andrew Hughes, Beverly Keller, Erik Neilson, Franz Gruber, Gunther Braun, Mike Danning, Peggy Neal, Shin'ichi Chiba

Synopsis:

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.