Never really bought dry sacks, as such, for backpacking and hunting. Use a lot of stuff sacks which are essentially the same. Tough nylon fabric, good coatings inside, and make sure there is tough double stitching. When we have to wash them, we do NOT dry in the dryer. High temp tends to break down the coatings. Packs tend to be fairly water resistant, and inner bags just help some more.
For total waterproof bags, we use roll top NRS Outfitter bags for rafting
I like to use several smaller ones, instead of a large one. Get in bright colors so if you set it to the side it is really visible!