Either a large, constrained cylinder of water or a large volume of open water (perhaps in the ocean) could be persuaded to form a vortex by pumping energy in. The body of water would be acting as a fluid version of a flywheel, storing energy.
If one then removed the input energy, would the vortex continue for a significant time or would it quickly lose all its energy by heating up the water? Would storing energy in the rotational kinetic energy of a fluid be horribly inefficient compared to pumped-hydro storage?