Vampires hating water is a pretty common old myth. You’ve got loads of classic movies and books documenting vampires being stopped by water (the 1972 Dracula film and Dracula: Prince of Darkness to name a few). But was this idea created by modern authors and screen play writers? Nope, this has been around for ages.
Water is believed to be a purifier, washing away evil and sin. In Greece, naughty undead beasties were “exiled” to islands (because they were surrounded by water see), which isolated them and kept them from leaving and eating up the living. In some places people thought that souls were thirsty, so offerings of water were placed near graves to keep the deceased from wandering in search of it (Author‘s Note: my soul likes coffee, leave that at my grave instead). A trial by water (similar to what they did to supposed witches back in the day), which was called the iudicium aquae, was used to see if a corpse was really a vampire. If the body floated, it was a vampire. Simple as that. It was these types of crazy ideas that birthed the belief that vampires are disabled or destroyed by running water.
But that is only part of vampire water phobia; you also have holy water which is a whole other deal. Holy water is water that has been blessed by a cleric, therefore made sacred and possessing powerful anti-evil properties. When it comes to the undead, it has many functions. It was (and still is to some) believed when used on a vampire it would burn like acid, leaving horrible scars (think the Anita Blake series). Holy water is also said to be able to detect evil. If poured on ground under evil influences the water will either boil or smoke. If the holy water is poured into an empty coffin, it will keep all vampires from laying there, the same goes for pouring it on a grave. Then you have movie/book created myths, like in the film Salem’s Lot, in which a bottle of holy water would glow if a vampire were near.
So to sum it up… vampires + water = bad!