What would water and fire do to the Nazgul?

That is the reason some held back and had to be driven into the Bruinen by the rear attack of Glorfindel. They knew their horses would die in the rocks and waves. But Glorfindel in all his "First Born" glory scared them and drove their horses into a panic, so they dived blindly into the water to be swept away.

The Nazgul did hold back, but they did enter the water and almost crossed. They didn't know of the coming tidal wave. Yes, they were afraid of water, BUT they dismissed there fears because of the drawing of the One ring. It was so close that they momentarily forgot of their fears.

I think that another reason why we Nazgul try to avoid fire and water is because of our cloaks. Without garments we can't do anything and it takes a long time to get a new one. So it makes a strange kind of sense that xwe'd stay away from fire which would burn our cloaks up and water which could sweep it off and away.

The garments and horses where only disguises they were given. If there garments were destroyed, so what? They unleash more fear when uncloaked. If they lost there horses, it was something that delayed them, more critical, but not something to fear water and fire over. Horses can swim!