Drinking water days/weeks before the U/A is of no help. THC is stored in the fat cells and is not water soluble; therefore you cannot "wash away" THC from the body like you can wash dirt from a car.
All youve succeeded in doing was that youve diluted the THCA concentration that was present in your bladder to the point where the urine sample registered below the cutoff thresholds of the drug screen. When consuming fluids, you are diluting. Dilution techniques are only temporary. Once all of the fluids ingested had all been urinated out, the "dirty" donor would resume testing positive for THC.

There is nothing that can permanently rid the body of THC except for time and fat-burning exercise.