A simple and reliable release mechanism for water rockets can be built from PVC plumbing supplies and cable ties, as in the diagram. This will cope with pressures in excess of 100 p.s.i with safety.
Step 1: General principles
An easily controlled release mechanism is shown, whereby the forces trying to drive the bottle upwards are directed downwards along the length of the cable ties, with only a very small component, courtesy of the bevels on the tie terminations, directed outwards against the retaining sleeve. The coefficient of friction for plastic on plastic is low and the actual contact area is reduced, because of the internal curve on the sleeve, such that very little effort is required to pull it downwards, well within the amount of force that can be supplied by a radio control servo, for example.
Step 2: Construction
The details are illustrated here. You may find it easiest to take a bottle to the hardware store and browse around to find bits that fit together without too much slack. The black 'O' ring is important in providing a tight seal otherwise the water will leak out as you pressurise the reservoir.
Step 3: Release
The outer sleeve is shown in the locked and released positions.
Step 4: Launch
Pump it up to 80 - 100 psi and pull the sleeve down to launch. A good trick is to get an unsuspecting person to do this from close range and they will be soaked by the escaping water.
Step 5: 3...2...1...WHOOSH!
Works best with the bottle between 1/3 and 1/2 full. Enjoy!