Drinking water does not "detox" you of THC. THC is fat soluble and not water soluble.
However, the THC metabolite concentration thats present in the bladder can be diluted by drinking fluids, thinning out the concentration to where the urine sample registers below the cutoff thresholds of the drug screen. This action is only temporary - once all of the water/fluids consumed had been voided out, a "dirty" donor will resume testing positive for THC. Therefore drinking fluids is only helpful on the day of test. Drinking water days before the test does no help. It may cause frequent urination, but it has no effect on the rate in which the kidneys filtering THC metabolites from the bloodstream. And as long as fat cells are being burned, THC metabolites will be released into the bloodstream, filtered by the kidneys, and end up in the bladder to be eventually urinated out.

So for 3 days before the U/A, you want to stop exercising and other fat-burning activities and go into "storage mode" - be as dormant as possible to prevent a bunch of THC metabolites from being released into the bloodstream, which will end up in your urine to be detected.

BTW, who is making you take a drug test?