The brothers of Nephi asked this question. Nephi explains that the water in the river was filthy, but that the fact had escaped their father, because of other things that had occupied his mind. He also explains that the water represents the awful gulf that separates the wicked from the tree of life, and from the association with the saints (v. 28). The gulf that separates Dives from Lazarus in Hades is in existence already on this side of the grave. Saints and sinners never endure the company of each other.
The water also represents hell, prepared for the wicked (v. 29) by the devil (v. 35).