Water dosent do anything unless its on the day of the test and the only thing it does is subsitutes water for urine (fat is not water solubal people) so ur pissing water and thats not generally accepted for a drug test but i have heard that it depends on metabolisim and shit also a high fiber diet will move the thc infested metobolites to your shitso unless its a stool sample that would be really helpfull
i was blazing some hash on a thursday and got tested that sunday negative (also always use midstream piss and never your first piss of the morning which mine incidently was)