It's a very common mistake to describe exterior plywood as "marine plywood."
DIY and building shows on television seem to call any plywood "marine plywood." Over the years “marine” has become a generic term to describe plywood that is exterior rated.
Standard exterior plywood is bonded using an exterior phenol glue. The specification for this is EN314-2 Class 2. Previously termed "WBP" (Weather and Boil Proof). Plywood bonded with this glue will not delaminate, even after 72 hours in boiling water.
Often, when a client is asking for Marine plywood they actually mean WBP bonded exterior plywood. This is a relatively inexpensive product and can be softwood plywood, tropical hardwood plywood and birch plywood.
There are other types of WBP plywood available but these are the most common.
Genuine Marine plywood is normally used for the boat building industry or where the board is going to be in contact with water and moisture for a very long time. The standard for marine plywood is BS1088 - bond specification EN314-2 Class 3. Above this is "Lloyds Approved". The glue that's used to bond the veneers together is exactly the same glue as in WBP plywood.