Model design question for pump rocket?

I'd like to create a pressured rocket preferably a water pumped rocket because unfortunately where I live I am unable to use solid fuel for the rockets. Is there a preferred or limit to how heavy the rocket could be? I don't want to use a bottle as a rocket as it is 'too simple' but I've been thinking of putting a extra layer on top of a bottle as it still ends up being the motor, or is there a alternative?