Human Torpedoes were used as secret naval weapons in World War II. The basic concept is still in use.
The Italian Navy experimented with a primitive tiny sub carrying two men as 1918 and this craft did have some success.
The first truly practical human torpedo was the Italian Maiale (nicknamed the “pig” because it was difficult to steer) used in the Second World War.
The Maiale was electrically propelled by a 1.6 horsepower (1.2 kW) motor in most of the units manufactured, with a top speed of 3 knots (5.6 km/h) and often required a travel time of up to two hours to its target.
Two crewmen in diving suits rode astride, each equipped with an oxygen rebreather apparatus. They steered the craft to the enemy ship.
The “pig” could be submerged to 49 feet, and hypothetically to 98 feet, when necessary.
On arrival at the target, the detachable warhead was released for use as a limpet mine. If they were not detected, the operators then rode the mini sub away to safety.