What would water and fire do to the Nazgul?

[Kalimac]

My question is just that. In the notes to "Unfinished Tales" Christopher Tolkien mentions that his father was not completely consistent with his descriptions of the Nazgul; how early on he didn't conceive of their fear of water, but then he emphasized that later on (they don't want to go into a river, they much prefer crossing over a bridge). When Elrond calls up the water to wash the Nazgul away it seems to put them out of commission for several weeks before they get back to Mordor and become re-horsed, so to speak. And obviously they fear fire, as we see in "Weathertop" where one man and four hobbits in various states of disarray manage to fend off FIVE of them just by using torches. So what did fire and water do to them exactly that made them so afraid of it? There's probably an explanation in something I haven't yet read, but if anyone has any ideas I'd really like to hear them. Thanks!