Wow, I could have used one of these when we were having 100+ degree days this summer! It's from the 2011 World Maker Faire in New York. Notice that they're not getting wet (or at least not much).
The explanation on YouTube:
The Waterfall Swing at the 2011 World Maker Faire at the New York Hall of Science. Towering steel swing set holding arrays of 270 mechanical solenoids that create a water plane falling in the path of its riders. Riders pass through openings in a waterfall created by precisely monitoring their path via axel-housed encoders, creating the thrill of narrowly escaping obstacles.
Check out more about the Waterfall Swing at the Dash 7 website.