True baltic birch is completely void free, very nice wood though it's considerably heavier than okoume. Use care though, there are lots of suppliers that call their wood baltic birch, when it's just cheap birch plywood. The other downside is it comes in odd sizes, think it's 60" square, so you have lots of scarfs.

You might look around and see if you have an aircraft supply house in the area, they often sell marine grade plywood as well as aircraft grade, either of which will work.

Also worth asking the big box lumber yards as well as the hardwood yards, most can order marine grade okoume, you just have to wait until they get it in, which might take a bit longer than usual.

Bill H.