Anywhere you can make a straight run, cleat the plywood on the underside of the deck
instead of vertically to the hull... Bonus points if you can ventilate from the deck
ceilings to behind the hull... all the way down to the bilge
and lockers. If the air can heat up on one side of the boat, and go through the bilge
out to the other side, you'll have a boat that breathes and doesn't rot
Straight runs make for a much easier installation
than trying to follow the contour of the hull with plywood. You can break most 8-10 foot runs into 3 1/2 to 4 foot lengths with a vertical plywood divider and a varnished strip on top. Button head
screws make it easy to remove the panels... Get the bevel right front and back instead of having to scribe in a fore-shortening piece of a basketball..
Going straight runs also makes it pretty easy to set a router to cut a slot or two the length of the plywood going back, for ventilation so you don't get mold. It is a bit more difficult to do that if you've got vertical battens along the way. You might want a cleat the length under the slot, or glue a doubler behind the slot before you route
it depending on your panel lengths.
Sealed areas on boats are bad news for rot
and mold, and most all the foam in a can products are worse... they take on water
, even though technically closed cell. Pull it out in 3 or 4 years and it is a nasty sponge.