If you just can't get marine plywood, then MDO (not OSB!) is usually an acceptable substitute. Any lumber yard can order it for you and it is not very expensive. It's also pretty decent plywood, though there might be some small voids. In any case, it's far superior to AC exterior plywood, and certainly is a much better choice than any interior grade ply (which, IIRC, is often the case with Birch).

And, a surprising number of far-from-the-sea lumber yards can get marine grade Fir plywood, so you should not hesitate to ask if you don't see it.

Good luck!