The Water Rocket Achievement World Record Association 2008 - This is probably old, since my link to the US Water Rockets site is one I bookmarked a long time ago and just revisited on a whim. The promotion:
Do you have what it takes to fly a water rocket over 1,000 feet? If you think you do, then you could take home a fabulous prize in the officially sanctioned WRA2 "1,000 Foot Challenge" Competition!
Here are all of the competitions. They include single and multi-stage vehicles, completely hand-built ones, and so on.