[Gordis]

Originally Posted by Ibrîniðilpathânezel

It also makes me wonder about the Nazgul and their aversion to water. While it smacks of the old tale about witches and vampires and their inability to cross flowing water, it differs in that the Nazgul can cross them; they just don't like to. I have wondered if this is because of their aversion to something as inherently clean and pure as free-flowing water, or if it still contains traces of Ulmo's presence, which they might well have reason to fear.

I think the last answer is the right one: Nazgul felt the lingering traces of Ulmo's power in flowing water and feared it.

Note that the Witch-King didn't fear water:

All except the Witch-king were apt to stray when alone by daylight; and 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- UT, the Hunt for the Ring

Now, why didn't the WK fear water?

He was undoubtedly a powerful sorceror, a witch, so, if the reason were simply that "witches can't cross water" as in old tales, then he would be more affected than the others, not less so.

It is unlikely that the WK wouldn't feel as much "aversion to something so inherently clean and pure" as his fellows did.

So - we are left with Ulmo's powers. The WK was clearly the most powerful of the Nine, and the power of Ulmo was almost gone from the waters of ME. The Witch-King was simply powerful enough to disregard what was left of old Ulmo to enjoy swimming