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At least twice in The Lord of the Rings: The Fellowship of the Ring, Ringwraiths cower away from bodies of water.
The second time (the ford of Bruinen) can be explained away by their fear of some Elvish magical trap through the enchantment of the water; but what about the first time, at the ferry? One of the Nazgûl could have easily made the jump onto the barge and quickly dispatched those pesky Hobbitsses.
A note in The Hunt for the Ring, in Unfinished Tales explains:
My father nowhere explained the Ringwraiths' fear of water. In the account just cited it is made a chief motive in Sauron's assault on Osgiliath, and it reappears in detailed notes on the movement of the Black Riders in the Shire: thus of the Rider (who was in fact Khamûl of Dol Guldur) seen on the far side of Bucklebury Ferry just after the Hobbits had crossed (The Fellowship of the Ring I 5) it is said that "he was well aware that the Ring had crossed the river; but the river was a barrier to his sense of its movement and that the Nazgûl would not touch the "Elvish" waters of Baranduin. But it is not made clear how they crossed other rivers that lay in their path, such as the Greyflood, where there was only "a dangerous ford formed by the ruins of the bridge". My father did indeed note that the idea was difficult to sustain.
This is the citation referred to in Royal Canadian Bandit's answer.
So your answer is that in-universe there was never an explanation given, whereas out-of-universe it's just a plot device.
It is common in folklore for evil or "unnatural" creatures to be unable to cross running water. For example, this is a traditional attribute of vampires: http://en.wikipedia.org/wiki/Vampire
Tolkien himself noted that this idea was difficult to sustain for the Ringwraiths. In particular, they would have had to cross the river Greyflood (which had no bridge or ferry) in order to travel from Mordor to the Shire. (I don't have the citation to hand, it may have been in Unfinished Tales.) He seems to have gone ahead with it anyway, in order to emphasise that the Ringwraiths were inhuman, undead creatures.
Ulmo = "He Who Pours" (a.k.a Lord of Waters) controls even underground waters in Arda. Ulmo, second most powerful of the Valar, totally opposed Melkor's (and thus Sauron's) program of dominating the Creation. +
The Ring Wraiths are as anti-creation as you can get. To cross into (as opposed to over) Ulmo's domain would unmake the Nine (cf. the crossing of the Greyflood and their loss of form in LotR).
+ Edit (and special thanks to user8719) The Silmarillion, Chapter 1, re Ulmo
nor has he ever forsaken Middle-earth, and whatsoever may since have befallen of ruin or of change he has not ceased to take thought for it, and will not until the end of days.
That's as clear as you can get that Ulmo's power still runs through the waters of Middle-earth, even in the Third Age. /Edit
Background
It's been noted in other answers that Ulmo's powers ran through rivers. In order to understand why Ulmo in particular was feared by the creatures of evil, one needs to understand how Valar (the gods) acted in the world and at what extent.
First of all, there are some events in the books that are very similar to the "Fall of man" in Christianity, which in turn explain why Valar did not act as actively as they could have.
In case of the elves, it is the oath sworn by the House of Fëanor, as well as the various Kinslaying events, particularly the one at Alqualondë, which eventually brought the Doom of Mandos upon them, cited if you follow that link. There are many obvious parallels to the Fall of man here where Adam and Eve were cast out of the Garden of Eden.
In case of Men, it is the Downfall of Númenor when the humans raised a fleet with the purpose of attacking Valinor. After which the fleet, as well as Númenor itself, sunk through the ocean.
These events form a collective, "inherited sin" of sorts, at varying degree depending on which people you happen to belong to. And because of these events, Valar were not as active in the world as they could have been.
And then there is an ulterior reason above beyond all of this, a masterplan by Eru Ilúvatar hinted at. During The Music of Ainur when the world is created, Eru allows Melkor to introduce discords, because it would make the final piece all the greater. Meaning, the various heroic and tragic tales wouldn't have been as grand and there wouldn't have been as many chances to do good, if there were no evil in the world.
Ulmo
Ulmo is the Vala of water, rivers and oceans (similar to Poseidon/Neptune if you will), one of the most powerful. Ulmo never abandoned neither the Elves nor Men even after the various sinful events described above.
It is a recurring event through the various books that all waters are watched by Ulmo and he is repeatedly "on the good guys' side", taking a more active part in events than any other of Valar:
You can tell that Ulmo had a plan for the start to help the people of Middle-Earth: all the above events are related to each other in a chain of events that ultimately result in Valar finally deciding to actively help the people of Middle-Earth by sending out a powerful army which defeats Morgoth in the War of Wrath, the final great battle of the First Age.
Evil creatures fear certain elements
Ringwraiths, orcs and other evil creatures are well-aware of the mythology and history of Middle-Earth. They avoid domains where Valar are particularly powerful, including:
So all servants of Sauron avoid crossing waters if they can help it. The orcs never built any fleets, supposedly for this reason (I think Tolkien said this explicitly at one point but I don't recall the exact reference). The only servants of Sauron who went by boat were humans, whom Ulmo may be more tolerant with.
Note about the Ford of Bruinen
In addition, the Ford of Bruinen in particular - the river that the Ringwraiths hesitate to cross when chasing Frodo and Glorfindel (Arwen in the movie), also marks the border of Elrond's sphere of influence around Rivendell. The Ringwraiths may have suspected this or sensed that something bad might happen if they crossed it.
And the Nine are temporarily defeated there, by a flood summoned by the combined powers of Elrond and Gandalf, each wielding an Elven Ring of Power. It isn't said explicitly that this was caused by the power of the rings, but the elven rings seem by their nature to mainly hold defensive powers, judging from how they are used in the books. It would seem that one of the main purposes of the ring Vilya was to hide and protect Rivendell (and Nenya is used in a similar manner by Galadriel in Lórien).
It is very plausible that it is a holdover of the power of Ulmo. See this forum thread for more...
The Nine kings were given nine rings by Sauron. These rings modified them into the creatures that they became, creatures of darkness and evil. Their form or bodies perished and they had to wear armor and cloaks in order to be visible to others.
For many years the nine kings used these rings, which gained them great wealth, prestige and power. However, the effect of the rings made their bodily forms fade over time until they had become wraiths entirely, and served only Sauron.
The Nazgûl were untouchable to mortal men, unless attacked with enchanted weapons.
Wraith means ghost, spirit or something out of shadow, shadowy.
They are the Nazgûl, Ringwraiths, neither living nor dead.
Since they cannot die, for they are not among living I will say that they can be "perished" or "ended", and use these word in this answer.
The Nine fear light, fire and water. They fear two of the key earthly elements of this world. They fear light and reside in shadow and cannot show a true form in this world. Hence the armor and cloaks which they wear. If we say they are the creatures of the darkness, than that statement would be true; but if we say they fear water because they are creature of the darkness, than that would be false for they also fear fire (we can associate fire with both good and bad). In the end some of them were "ended" by fire.
At this point, we can also speculate that they also fear the elements earth and air. Let me further explain this:
In LOTR we see that they fear forms of hot, cold and wet - from the above we can deduce that they also fear the dry form. They can only be "perished" by fire, or enhanced weapons (any of the above), but we can speculate that they can be "perished" by any of the above in an enhanced form. In truth, they fear all the elements of this world for they are the creatures of the shadow (the Wraith).
Furthermore there is another theory in which each of the elements have three properties, so we can provide a connection to the LOTRs "perishing" of the nine. Fire is sharp, subtle, and mobile while its opposite, earth, is blunt, dense, and immobile.
They can be "perished" or "ended" by fire. Which is Sharp, Subtle, Mobile, we can also find one of fire's properties in water Mobile.
So, in theory and as a connection to LOTR. Three of these properites need to be present in order to "perished" or "ended" a wright. We could see that in LOTR, but one or two of these properties can potentially harm it. Two of them seen in air and one of them in water.
From this we can deduce that they cannot be "perished" or "ended" by water but they can be harmed by it.
They seemed to be unwilling to cross running water, although they could if they had to. Their greatest weakness was apparently fire.
Additionally, the Nine cannot see like men do and don't like to walk and hunt in daylight, but at night in shadow.
The Nine could communicate telepathically. They do not see during the day as mortals do; instead they see shadowy forms. During the night they see many signs and forms invisible to mortal eyes; it is at night that they are to be feared most.
Crossing the river for them was a nuisance.
"The Nine are abroad again. They have crossed the River secretly and are moving westward. They have taken the guise of riders in black." —Radagast to Gandalf.
Provoked, the Nazgûl crossed the river to take the Ring by force from a weak and injured Frodo. However, the water, enchanted by Elrond and Gandalf, formed a great wave and swept the Nine away, killing their horses.
They crossed the river in fear, anger and disgust, but they knew that it wouldn't kill them in the end. It would mean the end to their horses (enhanced by evil, but still horses of this world) and their hunt, but not them. They knew they could lose this battle if they crossed it, but it was all that they could do at that point in time, for they are drawn to the One Ring like nothing else in this and their world (the world of shadow). In truth, they had to cross the river, they had no choice, for Sauron, and for the One Ring.
the Nazgûl were forced to retreat to Mordor on foot and stop their hunt for the Ring
After this, the Nine received improved new mounts that wouldn't stop them at rivers, the Fell beasts.
They were surrounded by an aura of terror, which affected all living creatures; their aura (called the Black Breath) could be toxic to those hapless enough to come near them.
This aura also protects them from the properties in elements, which I have talked about above in this answer. Their aura can poison water and pollute the air around them. Leaving them unharmed and protected.
They were physically weaker in the LOTR - Fellowship of the Ring, as also their aura was weaker. We could see that from the crossing of the water and small portions of fire when Aragorn drove them away with two burning sticks.
they would become vastly more powerful. However, it is unclear as to how. However, as Sauron's strength grew through the books, the Nazgûl became obviously more powerful. In the Fellowship of the Ring, the Nazgûl's cries were simply unnerving to the hobbits (this may possibly be explained because it was important that the Hunt of the Ring remained in secrecy so they might have diminished their auras, and they did not have the rings of power), and they appeared to be physically weak, as five of them were driven off by Aragorn with two burning sticks. Additionally, Gandalf the Grey managed to hold off the entire Nine single handedly on Weathertop. In The Return of the King however, their cries are powerful enough to send all but the most stout-hearted of Gondor's defenders into a state of helpless terror, and the Witch King in particular has become so powerful that he is a match for Gandalf the White (which of the two is the more powerful is not revealed).
Situation which you are referring to with the ferry, only happened in the movie and not the book. For in the book the Nazgul arrive later and find that there are no more boats left for them to cross the river on.
En route to the new house at Crickhollow, Frodo, Sam, Merry and Pippin crossed using the Ferry just before the arrival of a Black Rider, who was forced to go around to the Brandywine Bridge as there were no boats kept on the western bank of the river. (In the film version by Peter Jackson, the encounter is more immediate.)
Before my answer, I think it is important to remember that this is a work of literature by Mr. Tolkien (revered writer, who used his writing as an expression of his philological passion, and in an effort to backwards extrapolate the English language, and to create a mythology that was unique to England, based on "explaining" folklore and cross-cultural mythology around a central (and fictional) theme...it is not a scientific treatise on Unified Field Theory.
That said, the powers of the Valar have always remained very undefined. As an example, Osse anchored Tol Eressea to the Ocean bed with seawead, but Ulmo (his superior) could not uproot it. The Valar have been given (literately) nearly unlimited power at times, and very diminished powers at other times, so all of it is deus-ex-machina (including the "once-flat-world" premise). This needs to be viewed as a work of (amazing) artistic fantasy, not a work of scientific fact.
Having said this, water has a connotation of purity - all was pure before "Arda marred", but it is the nature of water to remain unspoiled, even in the face of Morgoth's Ring. Of all compounds of earth, water is the simplest, the most pure, the most basic to life. It equates with the goodness of nature, literarely. The ringwraiths, by contrast, represent that which is unnatural (wraith = writhe = anglo saxon word for 'twisted' or 'bent').
The role of the Valar in the 3rd age is not made clear...they did send the istari much earlier, but in a supporting role, and most of the Valar had stopped taking direct intervention into the affairs of Middle Earth (especially in the Age of Men, whom the Valar have never tried to influence, and when they have, it has rarely been for good - please don't ask me for citation, 'the lost road and other writings', i think).
Whether through direct intervention or not, Ulmo "came rarely to Valinor" and has always been more concerned, compassionate, and active than most of the other Valar. When asked, Tolkien didn't mention this in his letters....why? who knows? It's an obvious answer. Maybe his car broke down that day and it slipped his mind...what some of us consider cannon, he considered a book. What he did say (or create the idea of) is that the Valar put into the world (through their music) what measure of strength and quality they posessed - by this, I mean that Ulmo's spirit flowed through the water, and gave it its goodness, whether he was directly intervening or not. Just as Melkor (most powerful of the Valar) put his spirit into marring Arda (making the world an imperfect embodiment of his malice, but in doing so, weakening himself), so also did Ulmo give to water (and Manwe to air, and Aule to the bones of the earth, and Sauron to the 20 rings).
Point being that the purity of Ulmo (who most of all has always contested evil) ran through rivers. It is not said that the Nazgul died when they touched water, only that they feared it...because the sensed Ulmo's presence and it's purity - much like a man with vertigo does not die when he climbs a ladder, but he will avoid one. Perhaps their conversion to spirit (fea) from body heightened this, much as an elf (aware of the distinction between fea and body) is more attuned to it (as discussed between felagund and an old human shaman woman on the nature of spirituality, elves, and humans...again, i forget the title to cite it).
To summarize, I believe the ringwraiths has a strong hydrophobia because their primarily spiritual makeup sensed Ulmo's purity and feared it.
Let's be honest, after Melko and Manwe, Ulmo was the third most powerful being there ever was under Iluvatar. By comparison, the witch-king was a mortal man (albeit powerful and enhanced by his ring, but human, and his ring was one of 20, made by sauron, who is, by far, Ulmo's inferior)...do the math...a portion of Sauron's strength vs a portion of Ulmo's strength = a man afraid of crossing water.
I welcome supporting or dissenting opinions.
