Contrary to popular belief, there is no meaning attributed to the intensity or width of a line. Even synthetic urine controls (which contain no drug metabolites at all) can produce a faint line on an assay. A faint line is still a line, which is a negative.

Niacin is an ancient myth. It does nothing to help pass a U/A.

By doing an Advanced Search with the terms "drinking water useless" and "water soluble" , you'll discover why drinking a bunch of water in the days or weeks before the U/A is of no help.