Here's how to get a Water Stone in Pokémon Diamond:
1. From the Contest Spectacular:
* You can win a Water Stone as a prize in the Contest Spectacular in the Veilstone City. You need to participate in the contests and earn enough points to get to the higher ranks.
2. Trade:
* You can trade for a Water Stone from another player who has Pokémon Pearl.
3. After the Post-Game:
* You can find a Water Stone in the Valley Windworks after you beat the Elite Four. It's located on a ledge behind some rocks.
Remember, obtaining a Water Stone might take some effort and patience!