Don't buy from a big box store. Go to a lumber yard. Will get better quality plywood for same price. Being inside the boat you don't really have to go with Marine grade. I'd use 3/4 and seal both sides and all edges to protect from moisture.
You might want to think about filling the area under the deck with foam insulation, front to back. The boat won't sink and the insulation cuts down on a lot of hull noise. Cut holes..NOT DRILL holes in plywood in various areas all over and squirt in can insulation. Then replace and seal the "hole plug". Remember to allow for the foam to expand.
Texas Anglers Recognition Program
TPWD Official Weigh Station,77563
Coastal Bend Weigh Team, Cert. Weigh Master
USMC66'- Trout Support.com