Step 2: Rocket Rifle & Candy Cannon
My favorite for launching water balloons is the a modification of the "Rocket Rifle".
You can see how to build that step-by-step in this video:
This will also work with the "Candy Cannon" and no modification is necessary. It's ready "as is" and you can see how to build that in this video:
Step 3: Modify The Rocket Rifle With The Cannon Adaptor
The Cannon Adaptor is simply a 2' length of 2" PVC pipe, a 2" coupling, and some reducer bushings to bring the threads down to 1". Lastly, a 1", Close (short), PVC riser so it will screw into the sprinkler valve on the rifle.
I added some camouflage paint to match the gun.