well you've got forceware drivers to start, trying uninstalling then, restart, use driver sweeper, etc...
and pc gaming is now an afterthought with console ports, it's not what it used to be. you can't expect games to be optimized for pc anymore. dx 10/11 is still a fairly unsupported useless thing in pc games, still hasn't taken off quite like it should yet.
I'm not sure if that isn't what's supposed to happen. If you were to drive through a mud puddle in a race, you'd expect to spray mud, not clean water. Perhaps the DX11 version adds that bit of realism. I didn't remember Dirt 2 giving me anything that looked out of the ordinary, although I never played it in Dx9 and I didn't normally play from the cockpit view.
What do you mean by, "My DirectX drivers are up to date."? Did you reinstall them, or did you just check to make sure they are up to date? Just because they say they are up to date, does not mean they are functioning properly.
I just ran the benchmark, and it runs in 3rd person view, so I ran the same track in a test race in cockpit view. When I got splashed while going through water, it was brown mud that hit the window, not clear water.