How much water weight can you lose in 1 week by taking water pills?

Depends: Presuming no excess fluid (patients in heart failure for instance carry around extra fluid) you can loose quite a bit of water weight using diuretics as a large % body weight is actually water. Wrestlers sometimes will lose 20+ lbs with a mixture of sweating and diuretics in order to "make weight". This is not healthy however. If you want to lose fat (instead of water) eat healthy real food.