It could be done. We have autonomous underwater drones that can map the seafloor. There are a lot of problems. Technically, you need the resources of a nation

It could be done. We have autonomous underwater drones that can map the seafloor. There are a lot of problems. Technically, you need the resources of a nation-state to make stealthy ones that can operate at depth. There are no off the shelf systems you can strap together. Salt water and pressure destroys things very quickly. It wouldn’t take long for non-moving propellers to get fouled with debris. If you want a mine that launches a torpedo, I think that already exists. If you don’t need to operate at depth, a surface drone would be sufficient. If you’re attacking ships, aerial drones or missiles are better. If you’re not a nation-state, diesel-electric submarines have been around for more than a century. They are easier to build and will be for a long time. Even once you have drones, what are they attacking? The US has less than a hundred submarines. In my opinion, this is an advanced technology weapon as part of a combined arms strategy. Basically a more sophisticated torpedo.