new cushions are an expensive proposition "any way you cut it"
I spent a LOT recovering mine professionally, but used the same foam. Here's an idea: find some cheap used sofas in good condition with 3 or 4 inch foam and recut the foam pieces to the size you need using an electric carving knife. Then do the recover yourself if you can. DO NOT buy a cotton material for the boat. It will absorb odors. Polyester, nylon and synthetic or combo materials are best.