[LorenPechtel]

They specifically talk about putting them deep. There's only one thing to be gained by going deep: pressure.

It sounds like this is simply compressed air storage, except using seawater to contain it so you don't need a pressure vessel.

[stubish]

There is no air involved. They are pumping water out, creating a vacuum. Water rushing back in generates power. No need to compress air or have a hose to the surface.