“Along the beach” in its name reveals where exactly we have to look it for. This is fortunate, as this area is rather narrow, and you can’t see far ahead.
Once on the beach, there is not much to see – it is a rather barren place with not much life. Following the shore north, pass the Karnwasten dungeon entrance, and soon enough you’ll get to the point where one Reef Viper protects a shiny thing on the beach. It is our Karnwasten beach Skyshard.
Sand Kingdom Moons - Super Mario Odyssey Power Moons