Isnt the point to such a course that you end up with mud, water, sand and heaven knows what else in your shoes, all over your shorts, and everywhere else? I mean, there certainly isnt any trail-runner known to humankind thats going to keep your feet dry and clean when youre plunging through water up to your waist and mud up to your knees. Maybe somebody makes trail waders. Otherwise, your best footwear option is to find a water-sports shoe that works reasonably well as a runner. The Adidas ClimaCool Cardrona ($85; www.adidas.com), for instance, is built on a running-shoe last and has a running-shoe midsole, but also has drainage holes so excess water runs out fairly easily.
Otherwise, what to do? I might ask, are you simply running this course for your own edification and enjoyment, or are you really trying to beat the guy whos just behind or ahead of you? Because, if the former, then I have a simple solution: Carry a pair of clean socks in a pocket, or even just tucked into the waistband of your shorts. Maybe put them in a plastic sandwich bag. Then, when youre finished with the first two wet miles and have the final (drier?) eight miles to go, stop for 40 seconds and peel off the old socks. Shake the shoes out while youre at it, in an effort to get most of the sand and grit out of there. Youll succeed only partly, but at least the fresh socks should help keep the stuff from grinding off your skin over the final stretch.
If you dont want to stop, then the other alternative is to get a pair of waterproof socks. Rocky Gore-Tex Oversocks, available at REI and other outlets, sell for $50, and are made with stretch Gore-Tex panels so that they conform to your feet and are fairly comfortable. They have tall tops and a stretch cuff that would help keep too much crud from pouring down into them during the really deep, wet stretches. But, that said, these likely will not keep your feet completely dry while wading in water above the sock tops. And then you risk having a sock full of water, which wouldnt be very comfortable!
Personally, I like the spare socks option. That, or just tough it out and enjoy the mud.