Where is the closest water stone to the third gym in black and white2?

In Pokémon Black 2 and White 2, the closest Water Stone to the third gym, which is located in Castelia City, can be found on Route 4. Specifically, it is located in the sand area near the entrance to the desert, where you can find a hidden item. You may need to use a Pokémon with the move Surf or access the area to obtain it.