distance

Keep a recommended minimum of 5 metres (17 ft) distance, or 10 metres (33 ft) with a high-pressure launch, during pumping and launching.

safety equipment

Try to wear safety goggles/visor when you're launching water rockets.
A helmet isn't neccesarily a must, since ordinary bottles are so light that it normally actually doesn't hurt when they fall on your head.

pressure

Thin plastic 1.5L bottles normally explode at ±12BAR (170psi), re-useble 1.5L bottles normally at ±15BAR (218psi) and 18.93L bottles usually at ±6BAR (87psi).
Keep with launches well under the maximum pressure.

the launch

Perform a count-down before launching the water rocket.
During countdown, check if the danger zone is empty.
With large rockets, try to compensate the effect meda on the rocket's trajectory by ajusting the launch angle.

weather

Try to avoid launching with rain, lightning, snow or sub-zero tempratures.
Also do not launch when there's poor visibility (with fog, clouds, etc.), because it's handy to be able to follow the rocket with your eyes.

recovery

We recommend to use a recovery system when your rocket is havier than around 200 grams, or if it goes higher about than 90 metres (300 ft).
Parachute and glider recovery are both possible.
Do not try to catch the rocket, unless it's a light bottle or so.

launch system

Use a manometer for pressure indication, and make the system as reliable as possible.
Know how to fix it when there is a problem.
Tip: try to carry tools and spare parts if you're launching water rockets.

location

Try to launch water rockets in an open field, with enough space, and a good view on the rocket, without too much trees.
Ground has to be stable, so that the system can't topple over.

stability

Ensure the rocket is stable, use fins, and you could even let the rocket rotate, like a bullet.
Achieve this by putting the fins in a slight angle.