The amount of water is only one component to drag. The fins and the cone placement will also determine the amount of drag. Properly attached fins will prevent the rocket from wobbling. A properly centered nose cone will cause the rocket to fly straight. Cones that are off-centered will cause the rocket to veer reducing maximum height.
Opposite forces cause the thrust of the rocket.
The force of the air and water is downward against the launch pad. Therefore the launch pad exerts an equal force upward onto the bottle rocket. This is easily demonstrated by releasing a filled balloon. The elastic force creates high air pressure inside the balloon. When the valve to the balloon is opened the air moves to lower pressure outside.