Thanks to my high metabolism, I've past numerous drug tests after smoking pot several days before hand. I also drank water to no end, I actually felt the onset of water intoxication.
If you really are that paranoid, just be sure that the urine is most concentrated towards the beginning and end of the piss.
And the first piss of the day is the most concentrated, with the intoxicants becoming less traceable as the day goes along.
But don't sweat it, you'll be fine.