Shot with the 400mm f2.8 L lens, at 1/4000th of a second.
The autofocus speed of these L-series lenses is absolutely incredible. I never, ever had trouble keeping focus lock, and these guys were moving pretty fast. (Then again, even motorsport photographers use Canon, so you'd expect it'd be up to the task of a relatively pokey waterskier.)
And the 1D mark III is a joy to play with. 10 frames per second, at 1/4000th, makes anybody into a good sports shooter.