The Big Mouth (1967) on Netflix

While fishing on a San Diego beach, Gerald Clamson catches ... a sea diver! Even more weird, the "fish" resembles him. The man, who is not (yet) dead, reveals his secret to the peaceful angler: he is in fact a mobster who has cheated his associates out of their diamonds. What does not help Gerald at all is that the other hoods are persuaded HE is the double-crosser they are supposed to have done away with. Will he get himself out of such a tight situation? He will of course, but not without a little help from Suzie, the girl he only has eyes for!