Buy nice or buy twice.
This is, hands down, the best dry bag on the market. Yes, they're expensive. However, I don't mind the extra expense if it's the difference between a wet sleeping bag and a dry sleeping bag when I get to camp. Roll-top dry bags are simply not completely dry. Watershed bags are. I recently used this on a 12-day self support winter kayak trip through the Grand Canyon. While I used cheaper dry bags for some stuff, I always kept my sleeping bag, headlamp, and clothes in Watershed bags. Not only are they completely dry, they are also much more durable than any other dry bag I've used. These are overkill for backpacking, but I can't stress enough how much Watersheds outshine all other bags in situations where full submersion is a possibility.
The Ocoee is the perfect size for a kayak. I think it'd be hard to fit any of the bigger Watershed bags (e.g. Chattooga) in a whitewater kayak.