Some shopkeepers erroneously (or is it deliberately?) inform customers that Marine plywood is the same as BWR grade waterproof plywood. This is simply not the case. Marine plywood is a much better kind of plywood in which unextended (undiluted) phenolic resins are used for glueing the plies together, which makes it stronger.

Marine plywood is of Exterior grade and meant for extreme exterior uses. Its also better than the Exterior grade BWR (Boiling Water resistant) plywood that is used for making kitchen furnitures.
It is a lot costlier than Commercial ply and slightly more costlier than the boiling water resistant category of plywood.
It is much stronger than Commercial grade plywood because of the superior wood and glues used in its manufacture.