The exact location you want to go for is in the southwestern part of the Memorial District. Follow the water to the west side. The shard is actually not in the water, but on a small dirt area, right in the corner of the district. This area is just under the main district platform, on the lower level.
The part of the name that refers to a grave is still confusing me. I can understand the first part, as the river leads to this spot, but I didn’t see a grave. There are some doors just above it, maybe they lead to the some sort of mausoleum?
If you are up for more exploration in the same district, you can find the Memorial Armory crafting station in the northern part of the district, just west of the Imperial City Prison dungeon. You can craft the Armor Master Set there. As the name suggest, this is where you can craft the set that increases your max health, physical and spell resistance. The 5 items set bonus is quite interesting. If you have the armor ability equipped while wearing the complete set, it increases your max health by 5%, and if you use this spell, you also gain additional physical and spell resistance.
Sand Kingdom Moons - Super Mario Odyssey Power Moons