This shard can be found at the bottom of a waterfall. This is the waterfall in a northern part of the lake that is west of The Hollow City. It connects this lake with the lake inside Tower of Lies.
The most fun way to approach it is to jump toward it from a nearby cliff. If you follow the main road that goes from northwestern Hollow City, and turn north at the first crossroads, you are going to end up on a stone platform. There is NPC Gwilir that gives quest “Truth, Lies, and Prisoners” nearby. This is Tower of Lies objective quest. From his platform jump into the lake bellow. Swim to the bottom of a nearby waterfall and the collectible is there.
Collectible is at the bottom of a waterfall in Tower of Lies (mine POI).
Next to the shore, close to a large rock sticking out of the lake.
Collectible map location.
Sand Kingdom Moons - Super Mario Odyssey Power Moons