For every action there is an equal and opposite reaction. The force used to thrust the rocket upward is pressurized air. Air is placed into the rocket. The more air you put into the rocket the closer the air molecules are squeezed together. Therefore, when the flaps of the launch pad are released the air forces its way out of the bottle and the acceleration of the air causes thrust.
Force equals mass times acceleration.
Water contains 773 times more mass than air. The acceleration of the water and air downward due to pressurization is only slightly less than that of air alone. Therefore, by adding water the force increases dramatically. This becomes confusing for students because the rocket is not a simple system containing one measurable force. The force the water exerts downward creating thrust.