Here's why:
* Water Stones are introduced in Generation II (Gold, Silver, Crystal). Pokemon Black is in Generation V.
* Evolution Stones are often found in specific locations, but this varies from game to game.
If you're looking to evolve a Pokemon that needs a Water Stone, you'll need to find an alternative method:
* Trading: Trade with someone who has a Water Stone from another game.
* Find a Pokemon that evolves using a Water Stone: Some Pokemon, like Poliwhirl, can be found in the wild and will evolve using a Water Stone.
Good luck with your Pokemon journey!