Well, I'm not sure about the B+W filters if they have Water proof coating or not, I haven't tried them yet. Anyway, their web site states that MRC (Multi Resistant Coating) is dust and water repellent, so they should be (see:
Schneider Optics - Home )
Be carefull to get a WPC (Water Proof Coating) filter if You are thinking about Marumi - not all filters are water proof coated!
Bostjan