I'am a strong believer in Darwin theory of  "The Evolution of Species"
More particularly the part dealing with "Natural Selection"
In a nutshell: The stupids die first!  Therefore Ensuring that their genes drop off the genes pool.

What does that have to do with Water Rocket !
Well.. If for whatever weird reason you feel like kissing a pressurized rocket because it's your favorite.
And that it blows on your face.
Don't be surprised to find your front teeth embedded inside the back of your skull !...."Got the Msg !" ;-)
Fun apart .. Think about it !
We are dealing with a plastic thickness of less than 1 millimeter.
To contain 2 to 3 times the pressure of a car tyre ! 
So be aware of it at all time....

Take this hobby Very seriously ! After all "This is Rocket Science" ;-> 

So Please keep a few rules in mind...
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Use plenty of space..
Put the crowd behind the firing line, 
and the small kids close to you, or a grown up..(See note..)
Whatever you do ..Do it before you pressurise the bottle.
Have a pressure release valve of sort, in case something goes amiss and you have to abort.
Fire with the wind in your back. So the rocket goes away from you. 
don't launch "real" dangerous objects in the air, (remenber Newton law!) .
"Just use comnon sense.. And play safe !."

Note: little kids tends to be hypnotised by what is happening.
they do not realised the danger if the rocket is falling straight down onto them. 
My 3 years old almost got hit (only a meter away) 
by a falling rocket and did not budge even with me screaming at him.
So "stay close!"