The wetlands mentioned in the name of this shard is what you’ll find in this area. There is a lot of submerged land, with many tiny islands. The stump we’re looking for is easily found. It is on the same island where you’ll find the large stone tower with the circular foundation and rooftop, right next to the large mushroom.
The stump is close to the water, and with it, the island’s coast. If you come from the west, you can see its light from the distance without problems. It could have served as a lighthouse beam, if the water was any deeper.
In a hollow stump…
Sand Kingdom Moons - Super Mario Odyssey Power Moons