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.