Only use bottles that are made to contain carbonated ("fizzy") drinks, other bottles are not made to withstand the pressures used with water rockets, the rectangular shapes in many non-carbonated drink bottles are deformed by pressure. NEVER use glass bottles.

Assume if it can go wrong it will go wrong. Bottles may explode. Poorly designed or incorrectly used launchers may tip over. Release mechanisms may fail, causing launches to occur spontaneously, or for the pressurised rocket to remain stuck on the launcher. An abort procedure should be rehearsed. Disconnecting the pump from a long air-hose extending from the launcher will allow the rocket to depressurise, provided the launcher and air-hose do not have valves prohibiting air escaping. Use a system where you can depressurise from a distance.

Fizzy drink bottles are designed to withstand 12 bar, I would recommend only presurising your rocket to 6 - 7 bar. It takes some serious pumping to go beyond this pressure anyway. Smaller diameter bottles will withstand greater pressure, so are safer than 2 litre bottles at the same pressure. Damaged bottles may explode more readily than others. Nobody should be within a few metres of a pressurised bottle, this calls for a remote (eg. string operated) release mechanism. Exploding bottles are a rare occurrence if the above precautions are followed.

Falling rockets may descend at great speed, possibly causing serious injury. Rockets should be fired away from observers. Everyone in the vicinity should be observing and be aware of what is about to occur, launches should be announced. Don't rely on your recovery system working (i.e. the parachute might not open) for the safety of those on the ground.

Never, ever launch a rocket at or towards people or animals. Rockets can reach speeds of 200mph and above just after launch, even empty plastic bottles at that speed can cause serious injury.

Cutting bottles can be dangerous. It is safest to make a small cut with a safety knife, allowing a scissor blade to be inserted, then use short-bladed blunt-nosed scissors to continue the cutting where desired. Adults may use knives for most cutting, but with children it is best to provide them with a bottle which already has a small knife-made cut, so they may continue with scissors.