There are a few ways to get around this:
* Trade: If you have access to a different game (like HeartGold, Black, White, etc.) where the Water Stone is available, you can trade it to your SoulSilver game.
* Utilize the GTS: You can search the Global Trade System (GTS) for a Water Stone and offer a Pokémon in exchange.
* Breeding: Some Pokémon can learn moves that evolve other Pokémon with a Water Stone. You can breed a Pokémon with a Water Stone move and then use that move on a Pokémon that needs it to evolve.
Keep in mind that these methods are less conventional and require some extra effort. However, they can be a good alternative if you really need a Water Stone in SoulSilver.