I think your choice for decking is going to be way overkill, why not just use 1/2 inch and reinforce where you want to drill into by making seat bases and glass them onto the deck...much less weight and you won't run into clearance issues with the rest of the boat potentially. If you wanted to run thicker wood on the deck, I'd keep it to 5/8...maybe 3/4ths inch if I was going to over engineer it...
Someone bought a Glastron on this board that had a floor in it that was over 6 inches thick! Previous Owner ( PO ) just kept putting a new floor over the old one..the boat got so heavy that he added old tires to the splashwell to keep the water out, not to mention the 6 bilge pumps to keep it floating!
It's easy to want to over engineer what was there, but really, you just have to be very thorough in sealing the new wood to keep it from getting damaged again ( mostly making sure the mounting holes for the seats/ski locker are sealed before the screws are put in for final assembly ).