Endow Priority chart
Kunai Element (applies only to "Throw Kunai" skill) > Charm Endows > Item Endow > Sage Endow > Archer Arrow Element > Weapon Element
If you want "Throw Kunai" to take on an elemental endow (such as cursed water or aspersio) you need to use "Flying Fish" as your equipped Kunai (They are non-elemental).
Where do I purchase Flying Fish?