Hey that's actually a good point I haven't thought about so far. And as Precious already said it does make sense.
I guess we already found lots of possible reasons why the Nazgûl are afraid of water

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I think that we have just about covered all the bases

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"This is the hour of the Shire-folk, when they arise from their quiet fields to shake the towers and counsels of the Great. Who of all the Wise could have foreseen it? Or, if they are wise, why should they expect to know it, until the hour has struck? "

Yeah that's what I actually tried to say, too
Once again we can see how much there is to find in Tolkien's works and how many symbols/metaphors, or however you call it, there is

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I highly doubt Nazgûl are scared of water, at least not directly the water.
I think it is the elvish "good" power which drove them away from the water. As I remember, it was Elronds power which made the water rise, so I think it was the pureness of the Elves and the names of some elvish "godnesses" which made them flee, not the water.
The witch king also got off Frodo after he said an Elvish...well...prayer you can say...so it has something to do with the power of pureness...

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Come to the Dark Side...we have Jim Moriarty

Not sure if anyone brought this up, but what about the water being a symbol of baptism or which washes away evil [sin]? I know Tolkien wasn't keen in putting direct allegories into his tales, however I thought of this one on the spot and thought it was an interesting idea.

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hey you're right I never thought about that. That could be an aspect too, because it also says in the book that Mordor was a land that could never become normal again unless the sea washed away everything.

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