SN2001 Deck info

Graeme,
When the sole was replaced by a PO, do you know if the foam was removed or not? If it was removed, that may be the reason for the ply. A foamed hull can support the glass but without the foam, you need the ply.

Graeme,
What's going on with the ply that was installed previously?
Once you start digging, you may find some surprises!!

Personally, I'd go no foam with a CPES'd and epoxy glassed (both sides) ply sole.

If you are set on using foam to maintain your legality and all that jazz then I would not use plywood... itll just be something to rot. I wouldn't even use it under the front seats. Originally the material under the front seats was plain lumber nominal 1 inch thick. You would be better off using cpes treated 1x8s or 1x12s or something of the like under the seats than going with plywood.   As you can see by what you pulled up on your boat plywood next to foam is an invitation to rot, pervasive rot that festers through the plywood with ease turning it to mush. Don't do it I tell you...   douglas fir board stock treated with cpes would be better, or some coosa board or the equal with a layer of glass on both sides.   Some will try to tell you that if you soak the plywood in cpes and glass both sides that it will be ok... don't believe them, not next to foam - its evil I tell you, pure evil .   The rest of the floor simply glass over the foam as the factory did, even if it gets damaged at some point from anchor trauma or the like then you will be able to fix it quickly.