Coming from a northern climate, sledding has always been a huge thing for me, but you can only do it a few months out of each year and only if the weather cooperates.
So come summer, clearly the best watery fun came in the form of a Slip-N-Slide. Really, in terms of downhill fun, it has the benefit of not needing a sled or layers of warm clothes. Of course, there are also no snowdrifts to land in when you inevitably get too much speed and veer off course.