
It also reads that only the Witch-king and two others, who were driven by the lure of the Ring to close by, actually went into the water: the others were chased in by Aragorn and Glorfindel. And Christopher Tolkien states, again in UT.All except the Witch-king were apt to stray when alone by daylight; and all, again save the Witch-king, feared water, abd were unwilling, except in dire need, to enter it or to cross streams unless dryshod by a bridge.
My father nowhere explained the Ringwraiths' fear of water.