So you want to make a water rocket?
Water rockets are very fun but if used incorrectly the can be extremly dangours...
Even on the launch pad.
Step 1: pressure test the make of your bottles
Recently i was useing really cheap fizzy drink bottles (50p or 1$)....
it didn't turn out well.
most fizzy drink bottles should not lose and shape and pressure to 140 PSI
I would not by cheap ones but mabye well known brands as they are better.
Its better to be safe than stupid.
When initially pressure testing the rocket, fill it with water. If it explodes it will be less catastrophic.