Dry bags are way too pricy to plan on using for that kind of stuff-and once you get water in, you will never get it out. The bag will be so humid for the next 3 seasons all your dry stuff will be rancid when it comes out. Get a disposable water bag and a pocket backpack, don't ruin your 60-100 dollar dry bag over it. Watershed seems to be the top of the top of the line here in Idaho, and the serious river rats here spend 90 days a year on the water. Very durable, shaped convenient for packing on rafts and people like the closure system alot better. I have a NRS Bill's Bag, which can be a pain in the butt sometimes and other times I'm incredibly thankful for it's durability, reliable waterproofing and backpack straps. Often I pack my Mountainsmith Maverick full and outfitted, as I would carry it on a backpacking trip, and then stuff it into my Bill's Bag, so that if SHTF in a 7,000 foot deep gorge 300 miles away from the nearest hospital, I have some semblance of hope in getting myself out alive. They're really bombproof and get the job done, the watershed zipper is just alot more convenient than trying to manipulate welded PVC into folding along an even plane.
Dry bags are expensive, and they are designed to do one thing: Keep stuff dry. They are good at it, but you don't just go rucking around camp with sharp slivery pieces of firewood stuck in there for convenient tramping. When you're on the water you'll realize just how important it is to have gear that is 100% dry, and if you get any kind of dirt, moisture or whatever in there, all of your possessions will rot by the time you get to the takeout.
I've seen Watershed and NRS bags stuck in washing machine keeper holes for an hour at a time, and when they come out the contents are bone dry.