What drinking water does is dilute your urine, so there's less THC metabolites per ml in the urine. As was said THC is not water soluble, but the metabolites, which is what they test for, are water soluble.
So, if you knew you were having a drug test in a month, you should go on the keto diet, or the atkins diet (any diet, really) to lessen the amount of fat (and thc) from your body.