Granted, but you get it by accessing Sinjoh Ruins on Wednesday, while Arceus is in your party holding a splash plate. 5 specific Unown must also be in your party, corresponding to the letters w, a, t, e and r, each of them must also have water as their hidden power. Without following these precise steps the water stone can't be located. It's also the only water stone now.
I wish that everyone realized the greatness of munchlax!