When we say a plywood is marine grade means it is weather and boiling proof. In this case, marine plywood = wbp plywood. The wood panel can stay in exterior under rain for long time. Even the weather becomes dry and wet again, the plywood won’t delaminate.
Even the film faced plywood can be marine plywood. Why? Because many film faced plywood are produced with wbp melamine or wbp phenolic glue. The middle east countries usually name their film-faced plywood as marine plywood or marineplex.
No, the major difference between commercial ply vs marine ply, irrespective of whether the commercial plywood is MR grade is that the marine ply is completely waterproof and is manufactured especially for use in marine applications. But, even the high-quality MR grade commercial plywood cannot match it. they are only moisture resistant to some extent.
Marine plywood is costlier than the commercial plywood and is stronger than it too. Another factor that differentiates and makes the marine plywood superior to commercial plywood is the type of glue that is used to bond the plies. The commercial plywoods are bonded using urea formaldehyde with undiluted phenolic resin while the marine plywood is bonded using plastic phenol resin you get from phenol formaldehyde. This is a synthetic plastic resin that makes the marine plywood waterproof. This makes the marine ply superior when considering commercial plywood vs marine plywood