1. Do I have to use water? Why can’t I just use pressurized air?

Using just pressurized air gives too much thrust.
Using 2 L of water gives the best results.
Air is less dense than water so you won’t reach as high an altitude.

2. Is more water better?

Always.
Up to a certain level.

3. How can I modify the design of the rocket to increase the duration of the flight?

Use no fins.
Remove the nose cone.
Locate the fins near the top of the rocket.
Reduce drag.

4. What effect will the wind have on the way I launch the rocket?

The launch angle will be affected.
There will be no change.
The amount of water should be adjusted.

5. How will the wind affect the rocket after it is launched?

The wind will help the launch achieve a higher altitude.
The rocket may be blown off course.

6. How can I modify the design of the rocket to increase its chances of making a field goal or reaching a goal?

By decreasing drag and increasing stability.
By increasing drag and decreasing thrust.