Headed for the Mountains
Join Date: May 2014
Location: Vancouver Island, BC, Canada.
Everything becomes less effective when wet. I am sold on Down bags myself, even living right on the coast. Down bags are also becoming more water repellent (including the down itself), to the point where Rab tested a Nikwax treated Down bag by successfully floating a person in a lake with it. Personally I have found that Down bags with the same temperature rating as synthetics are actually warmer, in addition to being lighter, though this will vary between bags and manufacturers.
If you follow basic sleeping bag care, such as keeping it in a waterproof sack unless in the tent, airing it when there is sun, and using a proper tent, with proper pitching and good campsite selection (so you don't get flooded), you shouldn't have problems with keeping Down dry.