there's a whole subforum on it... check it out here: 420 Drug Testing
But to not quite answer your question, not necessarily.. How long ago did you smoke, how often did you smoke, and what's your activity level? All those things play a role in whether or not you'll pass... Plus, do you know what kind of test it is?
Drinking water does help. It also depends on how much you were smoking for how long and what type of test it is. There's a lot of good information on this site. If you get time check out the 420 Sponsor Welcome to I PASSED MY DRUG TEST .COM. Pass any drug test!! They has loads of information and if you need to talk to someone they offer a 24 hour phone line.