The basic principle of Buoyant Energy (buoyant hydraulic energy storage system) is based on the well-established technology behind pumped hydro energy storage plants. The major difference is the arrangement and location of reservoirs at such plants. While conventional pump hydropower storage plants consist of an upper and a lower reservoir, Buoyant Energy uses a buoyant platform.
The energy is stored solely through the potential energy of the mass of the floating structure. In order to store energy, water is pumped out (air is pumped in). As a result, the floating structure rises. In order to release the energy, the structure is lowered and the inflow into the smaller reservoir powers a turbine (turbine mode).
And they include a shout-out to Seasteading: http://www.buoyant-energy.com/english/areas-of-application/off-grid-energy-solution.html