I have a dilemma, and I know there is no definite answer for my question. You choose something and you go for it.

How important is that I get waterproof plywood for ribs and spine? SUP, unlike kayak, is totally closed and sealed. But, in case of damage, normal plywood could degrade over time. But probably not that quickly?

The thing is, I would like to buy marine grade plywood, but in my country it's available only from 9mm (3/8 inch) and more. And that seems a bit excessive, especially for the ribs if not so much for the spine.