Half-Life 2 - Airboat Battle
In this demo, Gordon is doing battle with a helicopter while running around in an airboat. There's lots of water here, plus plenty of pyrotechnics.

Once again, the XL beats out the GeForce 6800 Ultra and easily surpasses the 6800 GT, too, until we get to 1600x1200 with 4X AA and 8X aniso, where all of the Radeon cards stumble. There's probably some sort of driver bug causing the slowdown.