Unfortunately, it will be a little while before you can obtain a Water Stone. You won't be able to get one until you get the Dive TM. The Water Stone is is the Abandoned Ship between Dewford and Slateport Cities. You can also get Blue Shards underwater in the ocean outside of Lilycove City, which you can trade to the Treasure Hunter by Mossdeep Town for Water Stones, but that is also later in the game, and you can also find Blue Shards occasionally attached to wild Clamperl that you find underwater.