Screw a pulley down to the seabed. Run a cable from a float through the pulley to a reel on a winch driven by a motor-generator. (Could be on shore, in a boat anchored out there, or bolted to a wind-turbine post.)

Crank the float down toward the seabed when you have power to spare. Let the float reel back out when you need that power back.

For extra credit, attach a whole series of reels to one winch, with clutches, so one winch can crank down as many floats as you have reels, one or two at a time. (Floats, pulleys, and reels are cheap, motor-generators expensive, so it pays to share.) Maybe, instead of a winch and clutches, the reels are driven with hydraulics.

Size the floats so the winch is a catalog item, say <1000-ton capacity. Use as many motor-generators on winches as you need for the MW you must generate. Use as many pulleys/floats as you need for the MWh you need to store.

The deeper the pulleys are, the more MWh each float can store. Keep adding floats until you have enough.