Re: Worse than I thought
When you have a seam and no support the only real choice is to either back the joint with aluminum sheet metal or to cut the wood so as all seams fall along a support. I've seen lots of original decks done with no support on the seams of just tiny metal tabs joining the ends. On my Starcraft, I joined the floor sections from behind with sheet aluminum and sealed it all in with epoxy before laying it in place. No part of any wood was exposed to air or water when I was done.
I laid my sheets long ways side to side, that way I got the least flex since all of my lower supports on which the deck sat, ran front to back. I cut all my pieces to fit just perfect, then removed the new wood, flipped them over, routed out the area to be joined with metal and then screwed and epoxied aluminum to the joints. I then coated both sides with epoxy resin and did the top sides with glass mat. I installed the floor as one piece and riveted it back in place. It will out last me for sure. I used 1/2" ply so it wasn't super heavy and my boat was only a 16'. The complete deck panel ended up being only three sections in all as I used separate, removable panels at the rear to allow access to the bilge and under splashwell area. It would also have been impossible to fit that area in place as a single piece. (I ended the wood deck at the rear splashwell and installed a single sheet of aluminum diamond plate at the rear to both increase storage height and to prevent an chance of water contact on the rear floor, which is where the old floor was the worst).
You can also ask the local lumber yard if they can get you larger sheets, I know larger panels are available for the sign industry, I've bought 5x10' sheets of plywood before. I was lucky and had four sheets given to me by a buddy in the sign business, if it holds up on a billboard, it will do fine in my boat. It was actually an odd dimension but it was pretty close to 1/2" when measured.
