This verse is quite interesting on a couple of levels. The first is that Nephi provides a bridge between the apparent symbological difference between his father's vision and his own. Where Lehi did not at all mention filthy water, Nephi does. Here he explains to his brothers that it really was filthy, but that their father did not see it "so much was his mind swallowed up in other things."
The second point is that Lehi could miss it at all. Nephi's explanation is that Lehi was more attentive to other things, and simply missed this point. From this we may suggest that the visionary mode of revelation, while cinematic, suffers from the same problems of a modern cinematic production - much may be there which escapes notice because focused attention is not applied.