There's a section on the website on how to "free your event" from bottled water. It was super easy for Tri the Rez. We just put water in those big 5 gallon coolers and let people fill their own cups. Now, I know you're thinking that there's still cup waste going on and you're right. It would be best if people would fill a reusable water bottle with the cooler water. However, it's way more eco-friendly to transport 500 empty cups than it is 500 half-liter water bottles.
Check out their website then stop buying bottled water!