I had substituted 2 litre bottles for the 250ml bottles and found that the amount of thrust that the nozzle was producing was really only suitable for the smaller bottles of 1 litre or less. I was getting fed up with this poor performance and was toying with the idea of finding something else to do when I had an interesting thought. What happens if you take the connector off when the bottle is pressurised?
This one is even simpler to construct (you have already made the connector and a set of fins), but demonstrates the violent power of water rockets.
The model on the left (with fins removed) shows what you get if you simply up the size of the body. For a pressure of 6 BarG, the nozzle still produces around 12 Newtons. Clearly more thrust is needed and to get this, you need to make the nozzle bigger.
The model on the right (again, with fins removed) shows what you get if you use the neck of the bottle. This provides around 300 Newtons thrust (don't try to hold onto this one, it could do you some harm) and allows it to carry more propellant. In fact, this sized nozzle will let you launch a 3 litre bottle.
Half fill the rocket using the same methods as with the 250 ml rocket to determine just how much to add - measure the rocket, determine drag, computer model optimisation of weight of propellant and so on - launch as before - using the bottle top connector and pump - except that this time, you will be unscrewing the bottle top and not the adaptor.
Unfortunately for us, the soft drinks manufacturers have made sure that the pressure inside the bottle can be released as quickly as possible with only a small turn in the top - we see this as losing a lot of water before managing to get the top off and launch the thing. In order to combat this, I found it necessary to remove all but the last 1¼ turns in the top's thread therfore allowing the release of the prorellant with only a small turn. This works well although you still get soaked - this time more effectively. Perhaps you should think of building a launcher. See the pictures showing the stability during thrust phase.
Use fins and don't get in the way.
To demonstrate the power, half fill a 2 litre bottle, screw on a normal cap and invite the disbeliever to throw it into the air as high has he/she can making them note the speed, height and effort involved. Then launch it and see their face.