I spend a lot of time kayaking too, always looking for a better mousetrap. P.O.E makes a few duffels that may meet your criteria. Keep in mind that regardless of the zipper you get it won't be as waterproof as a roll top closure.

http://www.pacoutdoor.com/2007/index.cfm...&familyID=2


Or a couple roll toppers.
http://www.pacoutdoor.com/2007/index.cfm...&familyID=2
http://www.pacoutdoor.com/2007/index.cfm...&familyID=2

If you decide to go with a roll top duffel I would personally make sure that it has a purge valve.

Sierra trading post has the small zippered type POE on sale:
http://www.sierratradingpost.com/p/0,73281_Pacific-Outdoor-Dry-Duffel-Bag-Small.html


Another company you may consider is Watershed. They have a ziplock type closure up top, then you roll it up. That way while you're kayaking you could keep just the ziplock closed so you can access gear easily and if you anticipate rough waters or inclement weather you could roll it up for full waterproofness. (No purge valve though)
http://www.nrsweb.com/shop/product.asp?pfid=2890&deptid=1632