From Proverbs 5:15 comes "Drink waters from thy own cistern, flowing water from thy own well."
Translate that how ever you may but I think it is after all pretty clear. Your body does have its own filtering system in place. Yes urine is a waste product there is no disputing that but what if you were in a situation without any fresh water or water of any sort.
To consider drinking from "thy own cistern" for survival. Now not many of us are in this position every day but even when fresh drinking water could be contaminated with such things as beaver fever.
Humans can live off food for days, but will start to feel the effects of lack of water within a few hours.
Ask yourself, what would you do if you were in a similar situation?
Were the ancients correct?