fairing is a skirt surrounding the nozzle. Is it useful?
There are two choices here:
(a) include a fairing.
(b) use no fairing.
Go to the Water Rocket Simulator and try both
fairing and no fairing settings.
Remember to keep all other parts of the rocket the same: i.e.
air pressure, water volume, lift-off weight, size and number of fins. And choose
a good nose cone design. (A bad one may mask the effects.)