This idea seems to make a lot of sense. When the wind is blowing, you can use excess energy from an offshore turbine to pump water out of an underwater tank. When the wind dies down, you open a valve and water pressure will push water back into the tank, turning a turbine in the process which can generate electricity. This way, you can generate electricity whether the wind is blowing or not. You have the problem of maintaining equipment in the marine environment and transporting the electricity to shore, but since you have an offshore turbine you have presumably solved those problems. I don’t know that this idea is enough to make offshore wind turbines a great option where they otherwise wouldn’t be.

On land, it seems like you can accomplish something similar with a basic water tower. Use wind or solar energy to pump water up there when you have excess electricity, then drain it down to turn a turbine when the wind is not blowing. If you have a hill, you can put a reservoir up there and run pipes to it rather than building the water tower. Maybe you can use a your water tower or reservoir both for water supply and energy storage, but the turbine itself is going to dissipate some of the energy you need to create water pressure in your pipes, so there are some physics and economics to consider.