If you really want to bend your mind on what is, essentially, a gravity engine, think on this.
What holds a ship up in the sea is the same force that tries to sink it. Because the "weight" of the mass of water you displace, to create the buoyancy, is created by the gravity pushing down on the mass of water you are displacing.
Of course once you get into the water, things change. Creating buoyancy at 2 bar takes energy, which can be harnessed to give back energy when the buoyant vessel rises to the surface of the water.
The difference between an attempt to reset an above air gravity engine (lifting the weight back up again) and a below water gravity engine, is that things weigh less in water than they do in the air.
Flooding and returning to the start point is free, donated by gravity.
Creating the buoyancy is an interesting one, because there is the possibility to have reservoirs of force both outside the water (weights above) and in the water (chambers of buoyancy).
The question is whether the same rules of conservation of energy apply in a water borne environment where the very pressure which creates the forces you are using, donated by gravity, also affects the weight of the vessels you are using. The mass is the same, but the weight, mass being affected by gravity, is different.
Personally I think that smart engineering, materials and a whole load of effort could produce something which would deliver energy by utilising the multiple gravity fields of above water, in water and on water.
Now here's the kicker. It would deliver such a small amount of energy for such a massive infrastructure that you would be far better off dropping turbines in the ocean flow, with or without large anchored funnels and take the power out direct.
These ocean flows are not stopping any time soon and will deliver power for up to 20 years without replacement. The method has already been tested in the Scottish isles and is currently deploying in Wales and Canada.
These are mainly tidal devices, open ocean flow has the potential to deliver far more power, just as wind turbines have improved in performance for massive offshore as opposed to onshore.
Some links.
http://www.emec.org.uk/marine-energy/tidal-devices/http://www.emec.org.uk/marine-energy/wave-and-tidal-projects/https://www.novainnovation.co.uk/bluemull-sound