I redid the floor in a 76 SC ss Bowrider about ten years ago. Marine grade plywood 5/8 was going for about 100.00 a sheet then and had to be
Opted for exterior grade and filled all the voids with marine tex. After cutting and trial fitting the pieces I covered it with urethane then topped it off with marine grade floor vinyl. It was still holding up except where the water lays back behind one of the battery boxes. I also did away with the wrap around seat in the front and put in a removable pedastal seat. That move provided alot of storage I didn't have before. be careful when you strip it out as the hull may twist and will continuosly pop rivets. The guy that has it now says he replaces the rivets with #8 SS screws and Marine Goop if they are below the deck height/ waterline. good luck.
I have an '83 Starcraft CC 22' that I just replaced the floor on, and I used 1/8" aluminum, instead of wood. I think it turned out pretty nice , but I'm biased. Bought the sheets (pre-painted white) at Fazzio's for about $88 a sheet. Easy to work with, too. Cuts with a circular saw and used heavy duty pop rivets I got from McMaster-Carr. I have a T-Top so I used 1/4" diamond plate under the console to provide additional support. I can send pics of the project if you'd like. Just e-mail me.
MARJAM, In NE Philly right by 95 North can order it for you, I rebuild my transom last year with 2 3/4" sheets adn 1 3/8" sheet, it costs me about $250 for all 3 sheets.
You should do a little research on the internet about Marine Grade Plywood, there are different types based on the type of wood used and the guarantee of the removal of voids in the wood as well as the laminating process.
Also note, if you are redoing a floow on an older boat you could easily get away with a good grade plywood, what I would do is use 3/4" and coat it with epoxy resin, all the way aound, edges, everything. When you mount the floor, pre drill your holes and fill with epoxy resin first then fasten it, this will keep water from getting into the wood around those holes.
Just make a note where all your fasteners will be and fill the holes with resin, you should use thru bolting with a backing plate on everything you can get access to and use 3M 5200 or 4000 if it will be in the sunlight to seal around bolts and backing plates.