A "waterproof" duffel bag constructed of 20 ounce PVC with a zipper closure. Available in two sizes, 40 & 75 liter.
 
I purchased my 75 liter duffel as a waterproof day bag in my OC-1. The zipper seemed an ideal way to frequently access necessities.
 
Within the year though, the zipper began to leak. Just rinsing off the bag after a day on the water, or exposure to hard rain, resulted in water drops inside.
 
Another member's two duffels, one of each size, performed the same.
 
I must note that we kept the zippers clean and in the closed position when not in use as per manufacturer's instructions.
 
At a recent "Paddlesport" event, I discussed and even demonstrated the leaking bags with the SealLine rep by picking up a brand new bag from his display, closing the zipper, and squeezing. Right before his eyes and ears the air came hissing out through the zipper closure. His response was that the bags already cost $100, and upgrading to a truly waterproof zipper would not be economically feasible. 
 
So now I delight in visiting paddlesport shops and squeezing these bags to show friends and acquaintances how poor the design is.