In old European (maybe pago-Christian) ghost-lore it is always held that spirits cannot cross running water, as it is pure. Stagnant water can collect disease and foulness, but running water washes all these things away and purifies itself - it is thus seen as holy, evidenced by the fact that as springs and rivers are relatively rare they were worshipped by all European (and probably more) pagan civilisations as being inhabited by gods.
Thus, this idea of running water being a bit like holy water would have influenced Tolkien, as it comes from the same old Norse family tales which tell that Trolls trun into stone in the sun and that evil spirits cannot wander in the sun/are revelaed (even the Nazgul cast a shadow in the midday sun).
As for fire, could this be because they are more beasts than men now (not in intelligance but becasue of their barbarism), and that thus they are scared of fire as animals are? Certainly fire was even more revered in ancient times than water as yet another sustainer of life and purifier - could this 'holy' aspect scare the Nazgul? IMHO, I wouldn't be surprised if salt scared them too - though throwing fish and chips at them is less impressive that drowning them at the Ford of Bruinien. [img]smilies/wink.gif[/img]
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