The Polartec does not lose it's insulative capabilities when wet (The fact that I'm still alive to post attests to that) and it's well worth it, it's thickness does determine how warm it will be. You might want to consider hitting some of the mountaineering outfitters and look for a 200 weight stretch fleece top and bottom. It would be ideal to wear under what you have now, adds almost nothing to bulk, does not "clump up" and adds minimal bouyancy for the warmth provided.