Forest Water is an item in The Wind Waker.
The Wind Waker
Forest Water is only found inside the Forest Haven. Link can scoop it into a Bottle, but as soon as he leaves the Forest Haven, the water spoils and becomes plain water after 20 minutes (or 30 in the HD version.) Link can sprinkle it on each of the Withered Trees found on eight different islands within the Great Sea to restore them, giving them back their true state. After Link has restored all eight withered trees, the last one will drop a Piece of Heart for him. Surrounding the Great Deku Tree, Forest Water is the only thing that gives him the ability to live.