that's bullshit, putting tap water into your piss test cup will come up on just about any drug test. The chemicals added to tap water (Cl, F, etc.) are filtered in the body.
the only way i can think of that you passed is that whoever you had to do this test for, used a home drug test verison and fucked up dipping the stick in the the urine, somehow.... or maybe the chlorine in the tap water through it off, but most lab drug tests will spot added water in no time.
I guess your lucky, too bad you probably now have a job a shitty place that actually drug tests their employees.