Attacking ships on the surface of the water.
Step-by-step explanation:
The submersibles are a small craft that is designed to function underwater. Unlike the submarines though, the submersibles need to be attached to another, larger vessel, as they are not fully autonomous. Nowadays they have a variety of usages, such ocean exploration, oceanography, ocean archaeology, ocean videography, and for some people passionate about the oceans for adventure.
When the first submersible was designed though it had a totally different purpose. It was designed to carry and attach explosives to the ships on the surface. No one in the 18th century expected such a craft to exist, yet alone to attack, and while the first submersible was operating totally unnoticed, it was not able to do the mission it was built for as it was not powerful enough to move through the strong currents.