This is another video I took during our visit to Chateau de Cheverny in France back in August 2008. There were lots of White Wagtails running around the grounds and several were hanging out at a small pond where we also saw a Kingfisher hunting. The Wagtails would fly up a few yards and then seem to divebomb back towards the water, but before hitting the surface they would slow down and land on a wooden structure that was in the water and furiously tail-wag. It was like they were trying to act like the Kingfisher by divebombing into the water.
Besides that strange behavior I also noticed a couple of Wagtails running across a patch of lilypads:
If you click on the video you can watch a hi-res version on YouTube.