Now If you doubt this tale is so, I met that spook just a year ago. Now I didn't stop for a second look, But I made for the bridge that spans the brook. Cause once you cross the bridge my friends The ghost is through, his power ends.
Like garlic and salt, water is one of the common substances widely held to have special powers. Among other things, this means it can be used to protect against various creepy-crawlies and things that go bump in the night. Putting yourself across some water might be all it takes to save your neck.
Folktales are inconsistent on which creatures this works best, but it's generally best against vampires, ghosts, and witches and wizards. For almost anything else, it's still worth a try. But beware of bridges: depending on the tale, they might be able to cross these or not.
Symbolically, it makes sense for water to work as a barrier against certain monsters. Water is the source of life and so naturally works against the undead. On a practical level, it can also deter predatory creatures that hunt by smell, as water can misdirect or damp scent trails.
There are some variations as to what form of water will work. Almost always, the water must be moving, as discussed above. Rain or the ocean may or may not qualify, but rivers always do. Holy water, of course, packs extra punch, but is less frequently found about the landscape.
If water is actually harmful to these creatures, you can Kill It with Water. See also Healing Spring, Super Drowning Skills. For potential contrast, see Walk on Water, Super Not Drowning Skills.
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Anime and Manga
In Hellsing, Alucard at one point shows his considerable power as a vampire by crossing the sea (on a private jet), while drinking wine, in daylight. Seras, meanwhile, has to ride in the cargo hold in a locked coffin.
There's also a later plot point about the need to get Alucard on to an enemy emplacement on a stolen ship, as hampered by the fact that most means of approach would result in him being shot down and dumped in the ocean, rendering him powerless. They solve the problem by opting for a plane too fast and high for it to be detected and shot down before it's directly above the ship. This, however, was All According to Plan: since the plane gets shot down and the ships destroyed, Alucard is left stuck in the middle of the ocean while Millenium attacks London.
In Black Blood Brothers, this is a weakness some vampires suffer. Jiro can cross water without a problem, but Kotaro is severely weakened and can die if he is submerged.
Hazuki has this weakness in Tsukuyomi Moon Phase, though the only time it really becomes a problem is when she almost drowns in a hotel swimming pool with a whirlpool feature.
One Swamp Thing storyline features a group of vampires that have adapted to living underwater in a stagnant lake; Swamp Thing destroys them by agitating the lake and turning it into running water.
Dracula, Trope Codifier for so many vampire traits, explicitly could not cross running water except "at the slack or flood of the tide".
In The Lord of the Rings, when Frodo escapes a pursuing Nazgūl by ferry early on, it won't enter the river to chase him. Later, when all the Ringwraiths attempt to pursue Frodo across the river Bruinen toward Rivendell, the waters rise up and overcome them, drowning their horses and slowing them down significantly. This is because they are under the command of Elrond, who wishes to bar the entrance to Rivendell. In the film adaptation, Arwen achieves the same effect by invoking Ulmo, the Vala who rules over water.
Moreover, in The Silmarillion, it is noted that the creatures of Morgoth fear water, because of the presence of Ulmo, the one Vala who remains in Middle-earth.
On the Discworld, this principle is known, and mentioned occasionally by certain witches and wizards, but doesn't seem to have any real effect.
The same is true for the Myrddraal: they will avoid crossing water if at all possible, but will find a way if their quarry is on the other side.
In The Dresden Files, water is one of the most powerful magical anti-agents, and running water can cancel out spells easily. In one book, a villain is able to completely disable Harry from spellcasting by suspending him under a sprinkler.
In the original Legend of Sleepy Hollow, the Headless Horseman always vanishes while crossing a certain bridge. When Ichabod is chased by the spirit, he makes for the bridge in the hope that it cannot follow. It does; it's implied this is because it was actually his rival Brom in disguise.
According to Word Of God, this belief is the reason Harry Potter's adoptive relatives took him to a shack in the middle of the sea in order to escape the letters he was receiving. Unfortunately for them, there is no such rule in this setting. (Either that, or Hagrid didn't need spells to reach the hut; being so big and strong, he could probably just row there by himself.)
In I Am Legend, protagonist Robert Neville is testing various ways to repel and destroy the victims of the vampire plague that he has survived. Reading up on vampire literature, he tries to test the "can't cross running water" thing by setting up a makeshift stream in front of his yard with a garden hose. When night comes, a particularly-intelligent vampire named Ben Cortman notices this, then proceeds to mock him by hopping back and forth across the trough. Neville shoots him in the shoulder.
In the Old Kingdom trilogy, the Dead and most Free Magic creatures can't cross running water (unless it's bridged with boxes of grave dirt), and at least some of them are harmed by rain.
Averted but discussed in The Merlin Trilogy. Merlin frequently gets seasick, and it is completely mundane, the way lots of people get seasick. However, he considers it embarrassing, so when someone mentions it he tells them that wizards have difficulty crossing water.
In one of Andre Norton's Witch World novels, a woman revealed to her brother what he had gotten into by showing him he could no longer cross running water.
In The Last Apprentice witches cannot cross running water. When the Spook suspects Alice, who has been straddling the line between good and evil, of being a witch he makes her walk across a small stream. She makes it, but barely.
At one point in the Malloreon, the main characters are chased by a horde of flesh-eating creatures called raveners, which only flee when they reach the seaside, not daring to approach the open sea.
In the Scholarly Magics series, witches and wizards become ill on when travelling on water, oceans included.
Interview with the Vampire addressed this matter by saying that vampires don't cross running water because they're territorial and streams often serve as natural boundaries for hunting territory.
Mercedes Lackey's Children of the Night uses the same explanation, with the vampire dryly adding that one might as well say vampires can't cross major highways.
Once, in the Northland Series, Mikki can't cross water, despite not being a wizard, because the superstitious ferryman thinks he's a wizard and won't let him on the boat.
Darke Magyk in Septimus Heap and its creatures don't cross saltwater - especially flowing or tide-influenced one - well.
In Prince Caspian, it's stated that most Telmarines are afraid of the sea (which is ironic considering their origins). Caspian, and the seven lords he sets out to find, are exceptions to this superstition. The sea turns out to house some dangerous surprises, but the water itself is not actively harmful.
In Melanie Rawn's Dragon Prince series, Sunrunners (faradhi) cannot cross running water without becoming violently ill. Though some can, and it's revealed later in the series that they are come from another race of light weavers called 'diarmadhi'.
This is used as a plot-point in more than one of John Bellairs' young-adult gothic-horror novels.
In Night Gallery, "Death on a Barge", the vampire's father keeps her on an island because she can't cross running water so she can't escape.
In Being Human, Tully tells George that, in order to prevent him from wandering too far and possibly getting into trouble, he should try to find a place surrounded on a couple of sides by water, saying he can't cross running water while in wolf form.
Myth and Legend
As mentioned above, water is generally held in various folklore as proof against the likes of witches, ghosts and vampires, the idea being that water is purifying and thus can impede or dissolve unholy forces. This gave rise to the trial of water, iudicium aquae, such as used in witch trials (if you floated, you were a witch), which was also used in some regions on cadavers (if it floated, it was a vampire).
In Scottish folklore, the Nuckelavee could not cross running water.
There is a common urban legend, a variant on the archetypal Vanishing Hitchhiker tale, which tells of a couple driving along when they pick up a girl who is hitchhiking. She asks them to take her home and gives them the address. On the way there they cross a bridge. Upon arriving at the house, they look in the backseat only to see that she has vanished. They go to the house and tell the people there what happened. The couple who live in the house explain that their daughter died a few years ago and many drivers have picked up her ghost asking to be taken home, but the ghost always vanishes trying to cross the bridge over the water.
Meta case: the frequency of vampire myths in local folklore drops and thins out as you travel further west in Europe. In fact the majority of European vampire stories, geographically speaking, are to be found in the land lying between two major rivers; the Danube in the west and the Dnieper in the east (which covers Transylvania of myth and folklore, among other places). Famously, there is only a twenty mile gap in central Europe between the Danube to the south and east and the Rhine-Rhone system to the north and west. Effectively, these two major river systems cut Europe in two, leaving only a very small gap in between them for vampires to safely pass. France and Spain have some vampire myths but are more of a "werewolf economy". To cross to the British Isles involves crossing running water with a vengeance - the English Channel. Britain has next to no vampire myths. Ireland, another running water sea away, has fewer.
In 1st and 2nd Edition, vampires could cross running water, but if they were immersed in it for 3 minutes they were destroyed.
In 3E they could no longer pass over running water on their own, but could be carried over it in a container. Also, they were not destroyed by immersion in running water if they have a swim speed (in other words don't need to make a swim skill check when swimming under mundane conditions) before becoming a vampire.
Subverted in Warhammer 40K: a folk belief is that running water can disrupt sorcery, but a sanctioned psyker says it's BS. The fact that the final ritual to summon a daemon princess is taking place on a mining barge in the middle of the ocean kind of underlines it.
Mayfair's Chill. If a barghest is tracking another creature that passes over running water, the barghest cannot cross the running water until 24 hours after its prey does.
Lejendary Adventures. The Cunning Living Dead monster known as a peccant cannot cross running water except at a bridge.
Chaosium's supplement All the World's Monsters. The Daughter of Kali can only cross running water at night.
Populous: The Beginning uses this as a strategic element. While it's possible to make boats, contact with any body of water is instantly fatal for all beings because nobody knows how to swim. This includes the shaman, despite her near god-like, earth shattering powers. Players with quick reflexes and a good aim are thus able to use the weakest offensive spell (fireball) to launch the enemy shaman to a watery grave. The simple Bog spell has the same effect, turning a patch of land into an impassable deathground temporarily. These properties are part of what makes Terrain Sculpting so important in the game.
In the fifth Tomb Raider game you found a demon who told you to turn off a water mill or something because the water was trapping him.
Remilia and Flandre Scarlet from Touhou have this among their weaknesses. This was used as the trigger for the Extra Stage in Embodiment of the Scarlet Devil, when Patchoulli conjured a localized rainstorm to stop Flandre from leaving the mansion, which unfortunately also prevented Remilia from getting back home.
In Dominions 4, vampires get this is an explicit weakness. They cannot directly cross rivers, even frozen ones that ordinary soldiers can, decreasing their mobility. They can cross bridges, however; only directly walking/flying over uncovered rivers is forbidden.
Vampires in Tsukihime have great trouble with crossing water, even Arcueid, who is more of an elemental embodiment than a vampire. She can do it, but it's a moment of vulnerability and it's noted that she frequently simply teleports across large bodies of water despite this method of travel being quite a bit slower and harder than you would think. Other vampires can cross water as well, but it's something that seems to make them sick or weak. There's one vampire who actually lives underwater, but it's a bit of a tradeoff since Sumire's ability to tolerate water has made her weaker on land.
Gunnerkrigg Court has a variation on this. The Annan Waters (between the Court and Gillitie Wood) are stated to be an impassable obstacle for magical creatures, but we eventually learn it's not the river itself that's impassable. There's a ghost haunting the banks who attacks anything that tries to pass. Notably, this ghost has no difficulty crossing the river, but she is bound to its shores.
Castlevania RPG has the Sorceress overhear workers talking about building an aqueduct:
Sorceress: Did I hear something about running water?
Igor: Fear not, mistress, you will not have to cross it.
Sorceress: Cross it? I want to bathe in it!
Tales of the Questor features this, when Quentyn is being pursued by the Unselighe Fey, who he remembers are unable to cross running water. He learns quickly how little that means when the Fey have the power to instantly dam up the stream.
This, as a weakness of vampires, was a plot point in The Real Ghostbusters ep "No One Comes to Lupusville", which dealt with a feud between a clan of werewolves and another of vampires.
In another episode, "The Headless Motorcyclist", the Headless Horseman has been pursuing Ichabod Crane, as well as the family and friends of all of his descendants for centuries. The latest descendant, a woman named Kate, lives near a bridge for protection, knowing the ghost can't pursue her over running water, though she admits that's no protection for others the ghost might target.