go to lilycove surf of the east side go past the two swimmers looking at each other and dive at the first location you see, when you are down there go along and if you go quite far you should reach a dead end, go up to the surface and there is a shallow zone with a poke ball on it. Pick it up, inside is a blue shard.
Chat with our AI personalities
find a blue shard using Dive, and trade it to the guy by Mossdeep, or catch some clampearl, they hold blueshards too.
Underwater
Exchange Blue Shard with the Waterstone. You'll find waterstone in Abandoned Ship, too.
Near mossdeep ciry
I think he is near or in Mossdeep City.