The available pressure at 700m underwater is considerable. There's no equivalent reservoir of high-pressure fluid on the surface (the atmosphere is at a much lower pressure, obviously).
(The pressure difference in submarines is notably much bigger and more troublesome than the pressure difference spacecraft in a vacuum need to resist.)
So surely it's the high pressure at depth that's the attractive resource here? Pumping a gas into a cavern could offer a positive pressure solution, and I'm pretty sure that's been proposed already.
You get exactly the same pressure from traditional pumped storage of the same height. (Well, ignoring the tiny density difference between salt and fresh water)
(The pressure difference in submarines is notably much bigger and more troublesome than the pressure difference spacecraft in a vacuum need to resist.)
So surely it's the high pressure at depth that's the attractive resource here? Pumping a gas into a cavern could offer a positive pressure solution, and I'm pretty sure that's been proposed already.