Ash Hardwood (Mahogany would be the best because its lighter and very tight grain pattern, but its pricey$)to built the ribs(Stringer system) Marine epoxy paint after you built the stringer. Paint every piece as you install it sealing all sides.
Use 1/2 inch Marine grade Plywood for the decking sealing all sides as well. You can buy sheeting from marine stores on the web that are made of composite material. $$pricey again.
Build in any compartments that you want and then carpet. Marine grade carpet and marine grade exterior glue.
Send me a picture of your boat to www.billswoodwork@hotmail.com and I can give you a better idea of how much material you'll need, or measurements would work even better.