A water rocket is propelled by pressurised air forcing water down though a nozzle. This creates thrust.
If you took a standard two litre fizzy soft drink bottle and pressurised it to120 psi the rocket would reach about 100 or so feet.
But then if you took 2, two litres bottles and pressurised it to 120 psi again the rocket would go about 150 feet or so because the rocket has more air in it therefore creating more thrust. The rocket will only go 50 feet more because of the added mass. You can stop this by making a two stage rocket. A two stage rocket will work better because it would not have to carry the full payload on all of its flight. For Full guide Click Here from Instructables.com
PDF File from NASA for creating water bottle rocket: Click here to download