Wow. Check out this amazing site dedicated to hobby water rockets.
For most of us geeks, black powder rockets got more of our attention as kids, but water rockets, which use pressurized water as their propellant, have their own allure, especially when they’re taken as seriously as the craft on this site: multistaging, launch and recovery systems, on-board cameras, it’s all here. These babies get series air too, up to 500′