Re: Minor mods on my new Intex boat
Welcome Rakumi! You've got access to a lot of experience here, so don't hesitate to use it. I have had the Sevylor HF360 and now use the Intex Mariner 4, and I made floor boards for both. I wouldn't recommend much under 1/2 inch for either and I think used 15/32" for the Mariner 4, which fits my needs perfectly, which include brief occasional standing, and assembly/disassembly at the water's edge each time. A major consideration was the ability to hold two swivel seats.
I've never had the Excursion, but understand the floor is much larger, and so would probably recommend 1/2" or even slightly thicker plywood, depending on how much standing you intend to do. Weight of the material is another consideration as well as how many pieces you intend to have it made out of, and how, if at all, they are to be connected. Mine are not attached to each other at all.
I don't think you will find the PVC underneath of much value, and it might even stretch/weaken the bottom material.
Probably the best person to give you answers on this question is mrdrh88. He has lots of experience with mods on the Mariner 4 and, I believe, the Excursion. He is on the Mariner 4 Mod thread often. Maybe you could ask him there, to provide info here. Good luck.