: I know 1/2" ( 12.5mm) marine or exterior grade plywood is
: recommended for Yost's boats sections but I found 1/2 birch ply
: (wbp glued) exactly half price than marine ply.

: I also read the smaller sections can be built using 3/8"
: (10mm) ply, and save some weight there.
: Is it good also using 3/8" birch?

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.