I�ve read a lot about this and talked to the pros, and I think �the best� is probably Shelmarine BS 1088 Lloyds approved marine plywood. Absolutely no one has anything good to say about American fir marine ply these days, so you are talking some kind of imported ply made from �African� mahogany from the get-go, which almost certainly means Okoume, Sapele or Khaya. BS 1088 is a British standard for glue, waterproofing, number of laminations, surface flaws, etc. There is another standard called BS 6566, which is apparently pretty decent as to water resistance and glue, but doesn�t have as perfect or smooth a surface on the plys.
Lloyds means that someone has actually looked at the plywood and says it comes up to the standard. I think if you are looking for top quality beyond question, that�s useful. But if you find a supplier selling lots of BS 1088 Okoume that seems to know what�s up with boatbuilding, I think that would be as important as the Lloyds stamp.
It looks to me like there is a lot of 6566 �Meranti� plywood out there that is an Asian product. It�s about a third heavier than Okoume, so I wouldn�t use if for a small glued lap project, but I think for structural uses on larger boats it�s getting a decent reputation. Maine Coast Lumber (see below) sells it now, so it can�t be total junk.
Most glued lap boatbuilders use Okoume and seem quite happy with it, but I chose Sapele, which is a bit more durable (for a beachable boat) and has a beautiful, swirling grain as you can see in the deck pictures. It�s also a tad harder to bend, which made the garboards a bit tricky. For a painted boat, I wouldn�t pay the extra price of about $20 a sheet. I used mahogany for the structural members, which on a glued lap boat doesn�t amount to much.
Getting high-quality marine plywood ain�t easy. I am lucky to deal with Maine Coast Lumber. They are a Shelmarine dealer and very nice folks. I drove up to their yard in York, ME, as a family excursion and they let me pick the exact sheets I wanted, gave me a lot of free advice, and shipped the wood directly to my house, even though in Connecticut I�m just out of their delivery area.
Further south of here the Shelmarine dealer is Harbor sales, and the Shelmarine ad in Woodenboat lists some others. The one place I don�t know much about is Boulter. They show up at the boat shows, and seem to be a pretty good size outfit, but I have no personal experience. Looks like Edensaw and Noah's are also pretty active in this market.
For the small amount of mahogany you�ll need, if you can�t find it locally, try Wall Lumber Company. They ship $100 �UPS bundles� that are just right. There is a huge hardwood dealer not far from my home called M.L. Condon hat advertises in WoodenBoat, but when I drive in to get four boards, versus contractors loading up thousands of board feet to build the local McMansions, they get kind of surly.