I've never heard anything about not getting the discs wet. I mean, hell, it's a mountain bike right? Stuff's gonnna get wet and muddy. Sure, you need to clean and lube things after a sloppy ride, but you need to do that with regular brakes as well. If anything, one of the big advantages of discs is that they are actually better than cantis in sloppy conditions.
Otherwise, I'd do some reasearch on discs by various manufacturers to see if there are any that should be avoided. Also, make sure you won't have any weird compatability issues fitting the brakes to the frame/fork you choose (although everything is pretty much standard now, IIRC).