I'm a bit out of it on current plywood construction, but here is info from around year 2000.
Regular exterior plywood is generally laid up with urea formaldehyde glue. It is very good stuff and I have used many gallons of it, which may explain my behaviour.
It is waterproof although the plywood itself isn't.
Marine ply is generally laid up with resorcinol glue which is tricky stuff to use. I've used resorcinol on exterior boat work and sometimes in exterior house construction ( millwork).
Done correctly, it is not only waterproof but also boil proof. If you use this ply for a seat base, you could go ahead and boil it without hurting the seat base
Maybe not that important but it is the best glue to use below the waterline on any boatwork.
In addition Marine ply should have more laminations per inch than standard exterior plywood. The laminations for standard exterior plywood are at 90 degrees to one another.
With Marine ply the lams are in steps of 45 degrees which creates extra strength/ rigidity in all axis. If you are looking for plywood to bend around a curve easily, Marine ply will not be your best choice!
If you are looking for plywood that needs to remain flat over time, marine plywood is perfect.
The species is important. Douglas fir marine ply is the best stuff I've encountered.
Spruce plywood, even with waterproof or boil proof glue, falls apart quite quickly if left out in the rain.
After two weeks of heavy Vancouver rain on a new house frame sheathed in exterior Spruce ply the poor place had more boils than Charlie Sheen's man parts.
Marine ply made out of Douglas fir is the opposite, you can mistreat it for a very long time and it will remain useable.
We sometimes use sign painter's or Crezon ply in place of Marine ply. It has a similar layup to Marine ply but also has a layer of special paper bonded on to create a smooth paintable surface.
I don't know how that paper is made, but it's very durable stuff, even without paint.
Some Crezon doesn't use paper surface, instead it has bonded resin top layer for painting. This might be the toughest stuff of all. About 35 years ago we used that to build a 68' long work bench in our shop and it has stood up well.
I've also used it in exterior applications with good success.
It's made for it!
Glen