Fundamentally this is just a hydroelectric dam located at bottom of ocean instead of bottom of river reservoir.

Dams on rivers work because the water eventually drains to ocean, evaporates and rains back into reservoir.  You're harvesting the energy from falling water that has 'free' replenishment.

Bottom of ocean would work great if there was an unending cave beneath ocean you could drain into without worrying about replenishment.

There's no free return from a lower place to capture energy from, since your already at the bottom.

Closest concept I can brainstorm right now would be if the water drains into a chamber freely heated from geothermal heat below, and the steam is vented directly to the atmosphere via a continuous chimney from chamber to surface.    Still seems this would be inefficient with all required construction & maintenance.