>There are no submersibles capable of reaching the vents that are not large and expensive.
I think we both agree on that.
My point is that they could be made, if the goal is just monitoring. It could even be passive. No engines, no propeller, just a one-time way to throw the water out of the body to go up.
At a minimum, it's just a ball with a few video cameras, and a cavity that will hold the water, when it's going down, and keeps the air when it's going up. Even light is a nice, not must to have, because we're watching for other submersibles, which emit their own lights.
I still agree that it's not simple. I have not used this word in my statements above.