Compressed air storage really begins to make sense when the tank is deep underground, or deep underwater, so you don't need an overbuilt pressure vessel.
The very good idea of floating solar panels on a reservoir (cool panels are more efficient, and keeping them clean is cheap!) raises the question of what to do with excess power generated in the daytime. One alternative would be to pump water up out of another reservoir downstream, but there might not be one. So, just put a big air bladder on the bottom of the reservoir and pump air in.
Same applies for offshore wind power.
About half the energy you put into a pressure vessel becomes heat that you would not get back, under water. (But rock is an excellent insulator.) With enough spare water, you can draw almost the same amount of heat back in on discharge. Or, you might have a use for cold air.