Mac..
Do yourself a favor and use an acrylic exterior grade synthetic product. You can use a Sunbrella knock off if you want and it will be cheaper than th real deal. Cotton will almost never be dry on a boat, will get moldy, not stand up to UV and will eventually rot from moisture and extended UV exposure especially when used for cockpit cushions.I would also recommend using Pfifertex http://www.beaconfabric.com/vindex.html?cat82.htm on the bottoms so, when, and if, they do get wet you can store them in the cabin up side down and allow the foam to dry out.This is a labor intensive project and you'll kick yourself in the end for cutting corners and not using decent exterior grade fabric.What density is the foam you are using?