Welcome to the Magnetic Tumbler parachute deployment system. The idea is that a magnet rolls trough a tube when the waterrocket is in a certain state (preferably apogee). And that the magnet attracts another magnet that has a trigger attached to it. The trigger releases the parachute-bay door

I advise to take a close look at the website by George Katz. His Aircommand systems are truly amazing! And he has a detailed building instruction that I used as a base for my design.

This is called:

Happy Building!

Ruben van der Laan