: : A lot of people have suggested that perhaps Ulmo might have
: : something to do with the Nazgūl's fear of water, but Tolkien
: : himself never fully developed the idea (as he found it to be rather
: : weak himself).

: As far as the fear of water, Tolkien seems to have drawn on eastern
: European mythology in which it was believed that evil spirits could not
: cross over water. This belief was later added to the vampire (or Dracula)
: myth. It does seem a bit out of place in LOTR, considering the power
: given by Sauron to the ringwraiths. And of course the main difference
: was that the ringwraiths could and did cross over water, they were
: merely fearful. But that in itself seems odd: the purveyors of fear
: being afraid of anything. It also brings into question how the
: ringwraiths managed to get all the way from Mordor on horseback,
: considering the number of rivers and streams that must have lain in
: their path. [...]

I think it's entirely in character for Tolkien to have borrowed the principle of the Nazgūls' aversion to running water from the tales of Dracula, or from the same folklore as informed them, without necessarily exploring it fully. Though it may be a modern addition, I believe I have encountered the same in folklore pertaining to the British Isles.

(Nazgūl and) THE ELEMENTAL SPIRITS OF NATURE

The explanation of the connection of such waters with the Valar Ulmo is strong in that he could in-dwell in any water anywhere in Arda. However, even in Tuor's day Ulmo was coming to restrict his influence to the wide oceans because of Melkor's corruption and poisoning of the waters of Middle-earth. Ulmo's domain, however, is the sphere of which most is told regarding the spirits less mighty than the Valar themselves. Among the Maiar, Tolkien presents Ossė the Lord of Storms who rages against the shores of Middle-earth and Uinen, the Lady of the Waters whose tresses extend into every river and stream. Other spirit kinds are named in passing, of which Goldberry and River Woman may be taken as examples. I see the incident at the Ford of Bruinen as involving an awakening of the spirits of the river rather than as a case purely of direct elemental control on the parts of Elrond and Gandalf.

I see it as being the elemental spirits of nature -- however active, dormant or awakenable they may be -- that the unnatural Nazgūl spirits fear. Crossing water is likely to be a calculated risk on their part, akin to the calculated risk taken by the Fellowship in risking the ire of the hostile mountain spirit of Caradhras.

FIRE, THE FOURTH ELEMENT

In Celtic Britain men spoke of the three aspects of nature: Earth, Sea and Sky (as in the 'threefold oath': "May the earth swallow me up, the sea drown me and the sky fall on my head if I do not speak truly." The philosophers of Ancient Greece and the mediaeval philosophers who drew on their works considered there to be four elements, in opposing pairs: fire vs water, earth vs air. Though Tolkien never addresses these systems of organisation directly, both can be seen if one reads between the lines.

Tolkien presents the first working of the orma (the fabric) of Arda as the sub-creative oeuvre of the Ainur Aulė, Ulmo and Manwe, rulers of Earth, Water and Air respectively. Into their working Melkor, the single greatest sub-creator, added the harsh elements fire and cold (as used as weapons of destruction by his drakes). Cold, for all its perils to the Children of Ilśvatar, brought snow and ice, which in spite of Melkor's intentions proved to have their own rightness and beauty in the scheme of creation. Fire, he convinced the other Ainur, also had its place beneath the earth, but if it had a redeeming feature as a tool of 'creation', it was to be very much a two-edged sword.

I see Aulė the smith as being the Valar convinced by Melkor to include the fourth element of fire in the shaping of Middle-earth for the role it could play in the raising of the mountains. In the hands of Aulė of smiths among the Children of Ilśvatar, (more-or-less) mundane fire was also a means of goetically creating many things of wonder, but Melkor's own fires beneath the earth are notoriously used in the forging of the One Ring by Sauron. (Sauron, of course, was himself originally a Maia of Aulė's faction.) Fire is also later 'abused' in the hands of Sauron's technologists who wreak destruction on the walls of Minas Tirith, and those of Saruman in the smoking pits of Orthanc and who bring steam power to the previously idyllic Shire.

The Nazgūls' fear of fire could be a very normal caution in the face of losing the (flammable) robes that give them their physical form. Alternatively, it might be testament to the fact that fire is a raw, wild element of nature as hostile to their unnatural being as any other, in spite of being originated by the master of the Lord of the Rings.