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Since both the air we breathe and the water we drink are inert it is amazing that we can combine the two to make a water rocket more powerful than an A or B rated model rocket motor. The force that makes the water rockets lift off is the pressurized air that pushes on the water and forces it out of the nozzle. Since air on its own does not have much resistance compared to water, the maximum thrust is achieved as the water is expelled. Once all the water has vacated the fuselage the remaining pressurized air will shot out without much force.
For an optimal launch you can fill your water rockets with about a third water. The trick is in compressing the air. If you can compress enough air to blow all of the water out of the fuselage then try filling more water into your rocket. This is a balancing act since the more water is in the rocket the less space for air so you will need to compress the air to a higher pressure to still get the same volume of air into the rocket as before you added more water. Remember that a rupturing water rocket can be dangerous so be careful when pressurizing the aqua rockets.
Unlike dry chemical rocket engines, water rockets use just air and water. The main thing you have to worry about is having the rocket hit something or someone as it takes off. Another concern as I stated in the previous paragraph is that over pressurizing a water rocket can lead to a rupture or explosion that can send bits and pieces flying.
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