The major differences between marine plywood and exterior plywood come from the standards for plywood grades set forth by the American Plywood Association (APA). Plywood grades are not regulated by the U.S. Federal government but are a voluntary set of standards administered by the APA. Different countries have different names, but most standards are essentially the same as is the construction of plywood. To know which is best for a specific project, it is best to evaluate the cost, use, aesthetics, and overall goals of what is intended to be built.
Plywood is created by gluing layers of wood (called plies) together at opposite grain patterns with the top and bottom grain always running horizontally. The top and bottom plies are called the "faces" which receive a rating to the degree of smoothness and blemishes which exist on the wood. As the grade decreases, wood can be "repaired" and altered in order to create a full sheet without knots. These grades are referred to as "plugged" as often knots need to be plugged in order for the ply to meet grade requirements.
The center layer of plywood is referred to as the "core" and often a joint is placed down the center of the plywood to allow for expansion/contraction. Depending on the grade of plywood, core gaps increase in size and frequency. The species of wood, the glue used, sanding, blemish levels, and core-gap size are important aspects of the plywood's grade.
Exterior, is the grade given to any plywood which is bonded with 100 percent waterproof adhesive and is intended for permanent outdoor exposure. Exterior plywood must be graded a C or above, meaning knots and knot holes of up to 1½ inches are allowed. The species of wood can also vary and often different species are combined. Core gap size for exterior wood cannot exceed 1 inch wide. Exterior wood is good for most all wood needs and can be purchased to look almost identical to marine plywood at a much lower cost.
Plywood graded C-D or CDX is often mistaken for exterior plywood but is not approved by the APA for outdoor exposure. CDX plywood is considered Exposure 1 plywood. It can encounter high moisture content but needs to be sealed for complete outdoor use.
Made entirely of Douglas Fir or Western Larch, marine-grade is one of the highest designations which can be given to a piece of plywood and is considered to be "premium." In order to be marine grade plywood, the outer plies must be graded at least a B or better. B grading means that the wood may have some knots, but no knotholes. In order to be A-grade plywood, there cannot be any knots or knotholes present in the layer. The maximum core gap size allowed is 1/8 of an inch, and both outer panels must be sanded, Medium Density Overlay or High-Density Overlay. The durability rating must be Exterior and a fully waterproof structural adhesive must be used.