this problem is hard to solve, as the 2 other gravestone mods have the same problem.
if you die near water, the spot where the gravestone wants to randomly spawn might be in a liquid, thus is cannot spawn
you might say then why not just spawn in water? well, what if you died in a lava lake... you'll need fire potions and alot of luck finding the grave back.
but what about normal water? that shouldn't be bad? but what if you died in an ocean biome and you're grave is like 20 blocks under water?
what the graves then usually do is search for another spot to spawn, but further away. this can end up spawning the grave in some very weird and far away spots.
I don't think there's really a fix for this, but what the gravestones could perhaps do is add a waypoint to a player's minimap for locating the grave easier