Holy water is supposed to be water from one of the rivers Jesus was baptized in and blessed by a priest (at least the stuff you can buy online). It is also nearly any water blessed by a priest, and it is a great weapon against vampires. It just eats through their skin like acid, complete with melting and smoke. Not only that, it is also vampire version of hemlock, death by drinking. A vampire slayer should never be without at least a vial of the stuff..