Yeah, but energy density isn't really significant. You may get less energy out of a m^3 of water pumped to a reservoir than you get out of a m^3 of battery, but even the smallest reservoir has a usable volume that probably surpasses the volume of all batteries ever built. You can just keep pumping.
No, you can't just keep pumping. To build a reservoir at the scale that you seem to imagine, you depend on locations that provide a suitable topography. Very few such places exist.
