Hang weights from reels, underneath, for energy storage: reel them out to generate, reel them back up to store excess for later. Gang dozens of reels on each motor/generator, for economy. (Using just one reel per motor/generator is what makes mineshaft energy storage uneconomical.) Make the weights out of peridotite rock, which is very cheap and 60% denser than most.

Certain places are better for this than others. Offshore of Rhodes, eastern South Korea, Monaco, the Philippines have very deep water not far off shore.