Down packs down great and is very light. I use a compactor bag in my backpack to keep everything dry. I've been on lots of outings and have never had my gear get wet. I realize that accidents can happen, but unless you are being careless you shouldn't have wet gear.
If you know you are going to have to do some crazy water crossings, you might want to look at another alternative.
If you ain't havin' fun, you're doin' it wrong