One the other hand, if you wish to cover with a varnish equal, so you see the teak grain in the plywood, Okume is the way to go. But any 1/4 dg fir based marine ply or ext grade glue plywood would work.
That is basically what I found. Exterior grade plywood works... my boat didn't sink (has been going strong for 5 years). However, because of the internal voids, there is severe checking. The wood breaks down from the inside. Protecting with a layer of epoxy and painting won't keep this from happening. Maybe a layer of glass would. I think if you are going to spend the time to make a boat, spend the extra $100 and make is look good and last.