Thats interesting as I can find no references to a 'marine' grade of MDF, which by its very constructions would be unlikely anyway.
Do you have any details of it ?
Waterproofing
One of the main benefits of marine plywood is of course that it is virtually 100% waterproof. This is not something that can be said for medium-density fiberboard, which really isn’t water resistant at all. Marine plywood is great to use outdoors, whereas MDF is not.
On that note, marine plywood is not going to absorb water, and therefore won’t shrink, crack, or crumble due to water absorption, which is not at all the case for MDF.
MDF vs. Marine Plywood: Which Should You Use?
When it comes down to it, if you are looking for durability, longevity, and waterproofing, it’s always marine plywood that you want to use. This is especially the case for any outdoor purposes.