> Make gas at bottom of ocean, under pressure. Bring it up to the surface (low pressure) and run it through a turbine. That isn't what OP meant (see his reply)

> Make gas at bottom of ocean, under pressure. Bring it up to the surface (low pressure) and run it through a turbine. That isn't what OP meant (see his reply), but if the reason you create the hydrogen at high pressure is to reduce the cost of pressurizing it more, you would not want to reduce its pressure. But let's say you do want to -- the energy stored in the pressure, would extracting that be worth the additional energy cost of creating hydrogen at the bottom of the ocean and bringing it to the surface? > Plug that turbine into the machine making gas at bottom of ocean. Not exactly free energy but very efficient. Harvesting the weight of the ocean to help generate hydrogen gas.