Leapold, I'm sorry to be the barer of bad news but, in my opinion -- it's almost a 95% certainty that you will INDEED fail.
When was the last time you smoked?
If it was less than a week -- or even two weeks in the case of a habitual, daily smoker. If you smoked the night before you got the call about the test. Forget about it.
I'd forget about it all together. Don't overload on water -- you're wasting your time and its bound to be a painful, inconvienent experience.
Have a clean friend or a couple of bucks to spare?
USE SUBSTITUTION. Obviously you're worried ... and what's that saying -- better safe than sorry?