Browsing recently through the latest issue of IEEE Spectrum - my favorite nighttime reading - I came across of one interesting idea - underwater energy storage, developed and currently being deployed in a pilot project by a Canadian company Hydrostor. Having recently considered a pump back hydro storage for one of our clients, we came to a conclusion that the potential and difficult to accurately estimate environmental impact of a large scale project (it would not be economically viable on a small scale), its high cost and other limitations make such a project hard to justify.
An underwater energy storage on another hand largely mitigates these problems. One version is a flexible underwater balloon, looking not unlike high-flying air balloon, is convenient for small-to-medium size applications, can be deployed temporary and re-deployed with little disturbance to the fauna and environment at large.
An underwater energy storage implemented as permanent installation is more suited for medium-to-large size applications and can actually benefit underwater fauna by providing with the structure to build an ecosystem not unlike the coral reefs.