: As long as the plywood is made with waterproof glue, then it should
: be OK. Birch is considered to have little rot resistance, if
: that's an issue.
: 3/8" is too thin, in my opinion, but others have used it,
: IIRC. I'd double it up at the aft end of the cockpit, at least,
: as one's weight often goes there.

Thanks Dave.
Actually I meant 3/8 only at bow and stern sections (if not so wide) and 1/2 for the other sections.
The seller writes that ply is for internal use but also claims the clue is WBP (wheather and boilled proof).
WBP is not for "water proof" but i think even marine ply is not.
Half price is charming... I just doubt about the birch.