The Pure Water, as it appeard in the game manual.
Pure Water is an item from Super Mario RPG: Legend of the Seven Stars. When it's used on a ghost, regardless of whether it is a Boo, or not, it destroys kills the enemy instantly. If it's used on a normal enemy, then it has no effects. It appears as a blue bottle with a crossed ghost on it. It's not useful if you are not in Kero Sewers, or any other locations with ghosts.