Drinking too much water won't give you a postive. The only thing that will give you a positive is too much thc in your urine.
It makes a difference what time of day you take your piss test. If you take it first thing in the morning you're more probable of having more thc in the stream. It builds up in the urine while your sleeping.
A high fiber diet will help move thc through the stool instead of through urine but you need more time for that to have an effect.You coould be one of the people that retains thc longer than normal. If this is the case I would highly recommend coming up with some synthetic urine or as a last resort use somebody elses urine that you are SURE is clean and takes no prescription meds.