I've been told by the guys over at Bob Marriot's not to wear anything made of cotton as this is a material that does not wick away sweat or breath well when wet.. Wear fleece, polypro, polyester, or something synthetic that will wick away sweat. I bought a pair of fleece lined spandex underpants on sale at sports chalet.. These were made to wear under snow pants / windbreaker shell so you can sweat and still stay dry in them..

I wear them under my waders and it works out really well. Keeps me dry and warm. There are currently lots of good deals on this stuff because everyone's trying to dump all there snow gear. I got a 60% discount and payed a little over $20.00.

As for Waders.. I just got a pair of Simms breathables and they are awesome. Very comfortable (w/ the underpants) and keeps me very dry. I payed about $150.00 for them. I got my pair from Bob Marriots in Anaheim (Orangthorpe) but you might be able to find a better deal in eBay.. Good luck!