Re: type of wood needed
William, Some tips. Everything on a boat must be much stronger than it's landlubber equivalent. So if you were going to use nails fto assemble something at your house, use screws for that same item on a boat. If screws on land, bolts on a boat. Use heavier wood for boat parts.
BTW-I disagree about using paint. I recommend poly resin for sealing ALL sides of plywood for seats. The wood will be constantly exposed to moisture and never dry out. Paint will not hold up, IMHO. I always use dried out pressure treated plywood for seat bases, decks etc. I coat it on all sides with poly resin.