If you would like to build a more "high performance" launcher, here is one made from various metal plumbing parts soldered (sweat fitted) together. The same basic design could be built from PVC plastic parts as well.
The incorporation of a launch tube adds about 10% to the rocket's max altitude, because it gets things going before much water is lost from the nozzle. The reason for the "L" shape is so that the launcher provides its own self-supporting, sturdy base (for a tripod variation see below).
The threaded union allows for the easy replacement of the sport cap once it is worn out from numerous launches.
Entire Copper Tube Launcher
Detail of Copper Tube Launcher
Diagram showing the most important parts
Another variation of the copper/Bristol launcher--here mounted inside an old camera tripod