jacky in the box

Well there is a place called treasure island or something like that. I think it is around mossdeep. You need the item finder and there are alot of things around there and some might be blue shards. You also find the shards when you use dive.
But since you can buy water stones in Fire Red and Leaf Green, you can trade the stones with a Pokemon to get alot of them

espeonrules

To find Blue Shards and many other shards, go to Mossdeep or Lilycove and surf in the water to the east of Lilycove, or west of Mossdeep. If you don't want any wild Pokemon during your search, you might want to buy some Repels beforehand. Look around for dark patches of water, and then use Dive on the ones you find. Once underwater, if you see any empty spots in the seaweed or dark patches shaped like circles.....click A on them and you might find shards or other items. Usually, however, you have to follow some kind of pathway underwater and then resurface to find shards. Good luck on that. As for the strongest Water type, there really isn't one...it all depends on what moves you teach it and how you raise it. Kyogre is one good one, but so is Milotic. It really depends on what you think looks like a good Water type for you.