For a couple of years I was routinely travelling to an island where my gear went ashore throuh surf and was subject to wetting. I used a roll top dry bag to hold my pack, sleeping bag and other stuff. Although wetted many times, the water never penetrated the outer bag and my things stayed nice and dry. I got a noce big size that had its own shoulder straps - althoughrather primitive, they worked fine for short hauls. For long walks, I pulled my "real" backpack from out of the dry bag and stashed the dry bag.