If you want it to last the ONLY thing to use is a marine epoxy.
The way I replace decks. 2 coats all surfaces including the edges.
The floor will outlast the boat hull.
I can tell you this with complete certainty.
Paint is not water proof....not even close.
Your wood treatment products are not water proof. Not even close.
Polyester resin is not water proof. Not even close.
Vinylester resin is not water proof. Not even close.
Polyurethane is not water proof. Not even close.
You see the pattern here? These are all relatively inexpensive products.....That don't work!
And now I'm going to rock the world with this next statement.
Marine plywood is not treated with anything to make it resistant to rot. In fact some of the best marine plywoods are made from woods that aren't very rot resistant species. It's more about how the plywood is made, the glue, and to what standard it's made.
And another statement that may rock some back on their heels. Fiberglassing something is not waterproofing. Fiberglass cloth aint a water proofer, it protects from physical damage and adds strength to structure.
Boat soles (floors) typically rot out from the bottom up, not the top down.
Carpets, rubber matting, faux flooring, and improper storage all lead to constant moisture and trapped moisture.
The wood gets into a situation where it never dries. It may appear dry from the top side but it's not.
It's why most times if you have soft spots in a sole, more than likely you have stringer and transom damage as well.
If you got foam below the sole and a rotted sole.....your foam is shot as well. Please don't use foam in a can. There are insulation foams and there are flotation foams. Unfortunately flotation foams are, you guessed it, EXPENSIVE.
The epoxy is the only way to go. You can get a 3 gallon kit for around 150-180
Well worth the penny.
The key is encapsulation. You are creating a 99% water impermeable barrier. If you were going to drill a 1/2" hole in the barrier, you would first drill a 3/4" hole, fill it with thickened epoxy and then drill your 1/2" hole through that cured epoxy plug.
Your sole, transom, stringer...whatever will literally last forever given the right product and care .
So Jarrett Bay down in Beaufort makes one of the world's top sportfish boats......out of plywood.
Always use the right product for the application and there will be no problems.