In addition, there's the idea of using pressurized underground aquifers as an energy store (basically, using bending and lifting of the overlying rocks to store the energy when pressurized water is pumped underground). The pressure can be transfered to a clean seawater stream by a device called a pressure exchanger, which can then be fed into a reverse osmosis plant.

Constructing gas power plants costs a lot less upfront (~½) and uses almost 0 land by comparison. The site time for the construction should also be quite a lot less. So if you want X amount of power available in a certain place next year, new gas generation is a lot more straightforward.