The name of this one tells us it is on an isle. This is not really helpful, considering there are vast number of them in Vvardenfell. The largest number of small isles are located in the eastern part of the main map, just where we should find this skyshard.
If you pay attention to the eastern part of the map, you’ll spot a large formation of small islands in once place. Head toward this area, and its northern part. There, on the northernmost small island, is where you’ll find this piece of a skill point.
The “Sea of Ghosts” might refer to the vast number of mushrooms in the area. They really dominate the area.
On the small island.
Sand Kingdom Moons - Super Mario Odyssey Power Moons