Instructions
Bug Catching Contest Prize
Travel to Johto's National Park, an area north of Goldenrod City, on Tuesday, Thursday or Saturday.
Enter the Bug Catching Contest.
Travel through the Bug Catching Contest areas until you encounter and capture a rare Pokemon, such as Scyther or Pinsir.
Win first prize for your chance to receive a Water Stone. The prizes vary, so you may wish to enter the contest more than once.
Pokeathlon Dome Prize
Travel to the Pokeathlon Dome, a building west of the National Park.
Enter the contests until you earn 2,500 points.
Exchange those points for a Water Stone at the prize desk. The prize is only available on Tuesdays.
PokeWalker Method
Transfer a Pokemon to your PokeWalker.
Enter the Beyond the Sea course, which only appears after you use the Global Transfer Station.
Walk 5,000 steps to increase your chance of encountering a Staryu.
Capture a Staryu and transfer it back to your "Pokemon SoulSilver" cartridge. Staryu might carry a Water Stone.
Sea Cottage Method
Travel to Route 25 in Kanto.
Enter the sea cottage and talk to Bill's grandfather.
Show him a Staryu. He will reward your deed with a Water Stone.