I think it was the Italians who were the kings of midget submarine warfare.

Their midget subs were also designed like torpedoes. But they had two divers riding on the back of it. They would creep up on a ship and attach a warhead to its hull. Then the divers would scoot away and KABOOM!

These guys were the terror of the British at Gibraltar.

The Germans on the other hand had the midget submarine worst program. Their subs were unreliable in combat. There was one sub that was actually two torpedoes; one on top of the other. The pilot would sit inside(yes, inside it. Because it was carved to seat someone inside it with a glass bubble top so the pilot could see out) the top torpedo and operate toward a ship. When the torpedo underneath was in line with a target the pilot would launch the torpedo underneath. The only problem was that the release mechanism was poorly engineered. The result was the the pilot torpedo got dragged with the real torpedo and... well... KABOOM!

Definately a black eye for the Kriegsmarine.

Clark