there is one underwater on route 126. go to your AreaNav and surf until your face is directly above Sootoplis City. You should see a a swimmer around the dark areas where you're going to dive. Once you dive, head west until you see a small light square for you to head back up. Do not go to the surface. From the top right corner of that light square, travel upward and continuously hit A until you find the blue shard. (That's how I found it) There's no implication of an item being there, so swim around and keep pressing A. Good luck :)