Professor Nir Shaviv is a member of the Racah Institute of Physics in the Hebrew University of Jerusalem. He has also launched water bottle rockets with his son, and in 2006 posted about it on his personal blog ScienceBits.
Prof. Shaviv begins with a derivation of the basic rocket equation, using conservation of impulse-momentum. Then he adds the external forces, and later continue with calculating the velocity of the ejected water. This is achieved by considering the adiabatic expansion of the gas trapped in the bottle, and the work it does to accelerate the 'exhaust'.
It's all solid physics for a liquid and gas propulsion of water and compressed air. You don't have to be a rocket scientist to follow the derivations, although you should have passed the first semester of Physics for them. And if you have, and you are also in Seminole SECME, your technical report just became significantly easier!
