I would talk to the suppliers of the material and see if one could figure out the differences.

The reason for the use of marine plywood is the environment where it will be used will have such high humitity and sumbersion might be easier to deal with than material not submerged. Because of the increased humidity there is a greater chance of delamination, mold, fungus,incsects, etc even when not submurged. The glue that is used, maybe other treatments applied for specific issues like fire retardent, and the quality of the wood would all come into play.

You can buy baltic birch or doublas fir marine plywood. With the cost of those woods being less than tropical woods, it could bring down the cost.

I would think it should work for your pupose.

  I find the main difference is the marine plywood over other plys is that it has water proof glue and no voids. Regular exterior plywood has waterproof glue and one side knots filled with "footballs" but there are voids in the plys.  Birch plywood, used in cabinets, may not have waterproof glue and there are voids in the internal plys. The filling of voids and knots or use of plys with out imperfections insures uniform structural properties.  Other uses than marine applications are less restrictive until you get into building structure laminations. 

Both will rot if not protected. 

I talked to the supplier and he said that the Baltic Birch he has is made with exterior waterproof glues and it has "no" voids in it.  The notorious footballs have been changed into bats with wings and tails.  That said this Baltic Birch is sourced from  Russia where there is plenty of birch;  also it is nine ply half inch thick,$16.00 per 30x60 inch sheets--basically cabinet and drawer stock.  Quality cabinetry does not want voids showing on the edges. 

 It would a be useless debate whether quality  cabinets have plywood edges. 

Go for it. You just got to remember that what ever species you have to treat the fastener holes that go through the finish system. For example, if there is a screw hole so the seat can be removed it must be sealed inside so moisture does not enter under the head. OR you have to cover the screw with the finish system. IE paint.  For most of these builders it is epoxy satruation so I'd suggest the drill-fil-drill technique if you have fasteners that penetrate the surface system. I've used a marine caulk but that's ugly in a wood boat. 

Baltic Birch that I have been testing for several years now screwed to a fence post securing some fencing....And it has NEVER delaminated.....Had some soaking in a jar of water- still NO delamination..

And I built a 17' kayk out of 3 ply. 1/8'' ....A 5' X5' sheet a wooping $11.00 

   In more traditional boat building with plywood, mahogany plywood is used where the wood will be stained and have a natural finish.The rest is marine grade plywood.

The reeason for not having voids in the laminate sheets is that fiberglassed expoxy is not water tight. There will be water penitration into the boat's hull. You can verify this by talking ot sailboat class racers about how much there boats weigh at the beginning of the season compared to how much they weigh when measured at other times during the season. You should find the hull gains several pounds even if dry sailed. It is also why boats will sit in a marine yard awaiting repears.

   Voids can cause structural concerns too. 

I have justed finished multiple cuts--straight and curved and witnessed no voids at all.  This is high grade Baltic Birch that is used in high grade custom  cabintry--not the crap you get at your local cabinetry shop/ big box lumber yard.  Also, to reiterate the supplier said it is waterproof glue--he has to stand by his product or the word will go out that his ply is poor quality!   

I'm not getting involved in the wood type discussion, that's like giving advice on picking spouses, but I will say that any fiberglass and epoxy encapsulated boat that gains pounds of water weight in a single season is a seriously bad job of glassing and epoxying.

Laszlo

Laszlo, Speaking of spouses, I was thinking of trading my wife for some 1/4 inch marine plywood. How sheet of plywood do you think I can get?  

The only reply that comes to mind would involve a moderator, a lawyer and possibly an undertaker if I post it, so I am officially chickening out.

Laszlo

   The boats involved were Lighting class sailboat 19 feet long and a beam of  6 feet and a minumum weight of 700 lbs.The hull is all fiberglass cloth, epoxy resin and gel coat. so there is a lot of cloth through which the wate can migrate. 5 more pouds on a 700 pound boat is not much but for class racers this can be enough to change the tunning of the boat during the season.

   Most of the comercial built Lightning class sail boats used polyester resin  not epoxy resin . Polyester resin does absorb water. 

   But in either case water will still seep into into the hull and migrate through the cloth. With the strain of sailing the expoxy will develope micor cracks that will allow water to pass through and into the cloth.

One might not see this on a kayak but it is still happening.

   Has anyone tested soaking plywood w borax , then dry to reduce fungus and other critters,,, then coat with something.. epoxy, poly, or paint?

Thinking about a 14' x 5' sailing skiff, with fishbone skeletin to provide  a self bailing inner- liner. Seems to me it would be good to diminish rot etc. on the inaccssable insides.