Surf south east from Lilycove City, there is a house in the sea which trade many kind of stone from a shard! go to the house, you will see a man called "Shard Hunter", if you give him 1 shard, he will give you 1 stone. Blue Shard for Water Stone, Green Shard for Leaf Stone, ect... so, you can get a Water stone if you trade Blue Shard. Hope this usefull...