The best info I found in 5 minutes of searching is http://blog.beamtoothbrush.com/?p=428, which says that the research shows water flossing to be roughly as effective as string flossing.
There’s one confusing sentence in there that almost sounds like it’s distinguishing between water flossing and a WaterPik. I don’t have time to dig in further so I’m treating them as the same thing.