Drinking water wont do anything, it will only help to dilute on the actual day of the test, because THC is not water soluble water cannot "cleanse" your system of it.
I really doubt that one hit will cause you to test positive after 14 days. The best thing you can do is to get a home test, I think they are $2-3, and see how you get on with that, it will be a pretty good indication.
Your first pee of the day will have a higher THC concentration than the others, so don't use your first for the test, and drink a fair amount of water on the test day itself, but as you have hardly any THC in your system at all (if any) you wont need to over-do it with the water, cause if it's too dilute they will pick up on that and make you test again..
Don't even think about those drinks they are all scams, not heard one single good report about them, apart from dilluded people who take the drink and follow the instructions (to drink excessive amounts of water with it) and think the drink magically passed them, it wasn't, it was the water.