You can find this shard next to a large wooden bridge that connects Tanglehave POI island with an island south of it. If you are casually swimming in water around this area it is not that hard to spot this brightly shining collectible. For me it was, a sort of, a lucky find. It happened while I was swimming back to Bone Grappler’s Nest from Bloodtoil Wayshrine to die for the second time.
Sand Kingdom Moons - Super Mario Odyssey Power Moons