ch451,
Check out Watershed bags, I have several duffles and one backpack.
These bags are the absolute best investment I have made. Lifetime warranty, waterproof below 200 feet, awesome company to deal with. You put anything in these bags and when you see these bags these are hardcore. You won't have wet cloths in these bags, these are very tough and durable bags that seal, not that roll up and clip garbage. These are designed for water ops. I highly recommend, these are the bags that when you get one it's hard to buy only one. I have 4 now for many things, bob, camping gear, etc..
If you do get one, add the valve to it, it's worth it equalizing pressure at depth and conserving space while prolonging your stuff.

http://www.drybags.com/home.html