The same problem with height would also be an issue for the underwater thing. Quick googling suggests a very large container ship might have a draft of 16-17m when fully loaded. Probably you want the catenary at 20m depth then. How does a small get their pantograph (or whatever you'd call it) down to that depth, and then move forwards without the pantograph sticking 20m down creating a huge amount of drag. Not to mention how do you make the connection between the pantograph and the catenary in the (conductive) sea water?

Make the cable AC and the "pantograph" a (water tight) transformer that glides around the cable. I can't see any other way - DC is not going to work.

Just looking at a couple problems:

> Probably you want the catenary at 20m depth then. How does a small get their pantograph (or whatever you'd call it) down to that depth,

Does the connection from the 'pantograph' to the boat need to be rigid? Use a cable? Maybe it's not for the smaller boats which can't handle a 20m (probably more like 35-40m for the angle and slack) cable, but also need less power and thus pollute less.

> and then move forwards without the pantograph sticking 20m down creating a huge amount of drag

Presumably, the catenary's route would be cleared of obstructions - maybe even on a frame elevated over the floor, maybe at relatively constant depth - or do I misunderstand?