The only thing I've ever had actually work was drinking shit-tons of water 'til you're practically drunk off it and pissing every five mins. and taking a leak outside the building right before you go in. Probation can't take you to jail for diluted pee. Average employers usually won't turn you down either. But putting tap water in your urine is not a good idea. It will come back with a different temperature reading and chemical composition. You probably already took the test, but for future reference.