In The Lord of the Rings, the Ringwraiths are some of the scariest beings that appear. Their ominous, grim reaper-like presence adds a sort of horror element to the tone of the story, and they act as this looming specter of fear over the Hobbits. They spend a good portion of The Fellowship of the Ring hunting Frodo down, trying to get their skeletal hands on the One Ring. They won't hesitate to slaughter anything in their path in pursuit of what they want, and the shrieking screams of the Nazgûl just add to the overall frightening sensation that they bring.
Why, then, does it seem like they have a very big weakness when it comes to water? They are ruthless and unstoppable, at least until they encounter a body of water that they're seemingly unable to cross. Is there something magical in the water itself? Are they just really bad swimmers? What is the reasoning that J.R.R. Tolkien gave for such fearsome creatures being unable to handle a little bit of water?
In the film version of The Fellowship of the Ring, the Nazgûl stop their pursuit of Frodo and Arwen once the heroes cross the river Bruinen, which borders Rivendell. Arwen calls upon the waters to create a flood and stop the Nazgûl, and that particular instance would explain why they were hesitant to continue in the water, as it did end up sweeping them away. In the book, the same thing occurs, except it is Elrond and not Arwen who commands the river.
Allegedly, Tolkien originally toyed with the idea of making the Nazgûl afraid of the river as a result of being barred from crossing running water, which is an idea borrowed from classic folklore where evil things are unable to go across running water. Tolkien eventually decided against this, as he needed a way to explain how the Nazgûl got to the Shire from Mordor, as they would have had to get across the Anduin river, the Gwaltho river, and the Brandywine river. The fact that they are unable to cross the river Bruinen, however, suggests that maybe Tolkien didn't scrap the idea entirely. Even if the water couldn't kill them, as they cannot be snuffed out that easily, their physical shapes could be ruined by a strong enough force (like strong waves of water), and they likely wanted to avoid that fate.
Another possible reason for their fear is a more theological one. When Arda (the world) was first being formed, one of the Valar, Ulmo, came up with the idea of water and created it. Ulmo was also the Vala connected to Elves and Men in Middle-earth, and was the one who aided them the most. The Nazgûl, as servants of Sauron, feared the power of Ulmo, and believed that some of his power might still flow through the waters of Middle-earth, hence why they avoided it as much as they could.
There is also an exception to this fear - the Witch-King of Angmar, aka the Lord of the Nazgûl, is the only one who isn't afraid of water. In Unfinished Tales of Númenor and Middle-earth, it is stated that "... all, again save the Witch-king, feared water, and were unwilling, except in dire need, to enter it or to cross streams unless dryshod by a bridge". Why is this one Ringwraith okay with moving water? There is no solid explanation, but it is theorized that he might have once been a Númenorian who was well-acquainted with the water before he became a Wraith, or that Sauron had given him special powers that allowed him to overcome this fear of water. The Witch-King is also the only Wraith who did not stray during daylight hours.
In Unfinished Tales, Christopher Tolkien (J.R.R. Tolkien's son) acknowledged that "My father nowhere explained the Ringwraiths’ fear of water", and explained that a total aversion to water would be hard to sustain because of the beats of the story that needed to be completed, as well as where the Wraiths needed to go. It seems that they were able to make a crossing if they were in "dire need", but generally preferred to avoid it.
Like many things to do with Middle-earth lore, there is no concrete explanation given by Tolkien, so a lot of these ideas are just conjecture and fan theories. Tolkien's world is incredibly expansive, but it also left fans with many more questions about the lore than he was able to answer in his lifetime. The exact reason for the Nazgûl's aversion to water is not totally clear, but it likely has to do with the Ulmo explanation or more of a general fear than an outright inability to cross moving water. Whatever the reason, it goes to show just how much this story has stuck with people that they're asking even the most minute questions about the way this world works many years after the story's initial publication.
