The shard is well hidden inside the dungeon. It is located within a flooded cave tunnel that connects two of the largest rooms. Inside this tunnel you should notice a small, hidden room, just beside the large metal door. The shard is inside this room.
Find and go through this tunnel.
It is found just north from the nearby Wayshrine.
Wansalen dungeon entrance.
Sand Kingdom Moons - Super Mario Odyssey Power Moons