Water only dilutes the drug metabolites. You need to only drink about 8 glasses of water a few minutes before the test. Cranberry juice is only recomended because it makes you piss. THC(this is in weed) is only fat-soludable. Detoxs do not work, The water that you take with them do all the work. Check this out.

http://www.cannabis.com/usage/dtfaq.shml