There are a number of ways of releasing a parachute - you
can do it electronically by: using a remote control of some
type; using GPS to detect an appropriate altitude; using a
means of detecting air pressure to identify apogee; detect
the time after launch to release; or, you can use a traditional
mechanical method soch as the 'Nose Falls Off At Apogee'
('NFOAA') method, 'Speed Flap' release or clockwork timer.

Some of the electronic methods mimic these but using
electronic methods goes against the 'available to everybody'
ethos with water rockets.

To determine the delay in opening the chute, fold the chute
up and then run with it, and see how long it takes to fill up
with air.