This is a sling shot we made to avoid having to use 3 people to launch a typical water balloon launcher.  We used PVC tubes as the uprights and sized them appropriately so they could slide over the support legs of our dock.  The band material is latex tubing which is available at most hard ware stores. The pouch is made out of duct tape. We drilled holes in the PVC in order to attach the tubing to the uprights.  We put rubber grommets in the holes to keep the sharp edges of the PVC from rubbing on the latex. I wish I had more close-up pictures of the uprights and the pouch, but I don't.

The picture above taken at night is a long exposure shot of a tennis ball with 3 sparklers coming out of it being shot into a lake.  Our original idea was to shoot a flaming tennis ball, but we settled with sparklers.  We also tried glow sticks wrapped around the ball and trailing the ball like a tail, but the glow sticks were not bright enough and could not be seen through the camera.

This sling shot will shoot a tennis ball about 125 yards and golf ball closer to 200-225 yards. Unfortunately though, the golf balls cannot be retrieved in the lake (because they sink) so we rarely shoot them.  Everything else that we shoot is retrieved. Usually a few of us go out onto the lake in kayaks, and we try to catch the tennis balls as they are launched.

Extra-Mini-Instructable: In the daytime picture above, you can see a fitness trampoline at the end of the dock.  We built a frame that sits on the lake bed and supports the trampoline so it sits level with the dock surface. The frame isn't very rigid because it is over 6 feet tall and built from extra pipes and hardware, but it is still really fun to launch yourself into the lake with the help of a trampoline.