Did anyone happen to see The History Channel program ‘Suicide Missions’ that dealt with mini-subs? It seems that every country eventually worked on developing some kind of mini-sub. Most of these were rushed in development and weren’t meant to be suicide weapons but that’s the way a lot of them turned out.
IIRC the British developed a craft that looked like two torpedos one above the other. The top one was designed to carry occupants in the vicinity of the enemy ship. They would then fire the real torpedo on the bottom. Problem was is that the release mechanism didn’t work too well and the top part with occupants was often carried along as well.
Sorry, besides the story of the four mini-subs deployed during the Pearl harbor attack (fate of three unknown, one captured when it washed up on shore) I don’t remember what they said about the Japanese.