As Mike indicated, the lack of voids in marine grade is one of the most important difference between the two. Marine grade will also have more (and thinner) laminations. This adds strength to the marine grade, but you shouldn't need the additional strength for this application. At one time, the glues were different, but as long as you buy an "exterior grade" plywood, it will have the same waterproof glue today. And some marine grade plywoods, of courese, are made of mahogany (as was most of the ply used during the original construction).
Neither marine grade nor exterior grade should delaminate with limited exposure to water, but either will delaminate with extended exposure to water. So, I see no advantage to the use of marine grade, unless you are purchasing mahogany, to be varnished (which you are not). The key to avoiding delamination, of course, is to make sure that there is nowhere for the water to get in. When you do this job, you should probably re-bed your ports. If you choose to use the Parkland product I suggested, and if you put it over new ply, and it your ports are properly sealed inside and out, and if your ports have good gaskets ... there should be no place for the water to come in. Even if you forget to close the ports from time to time, you should still not get any delamination, provided the trim at the bottom of the panels is sufficiently sealed where it meets the dripledge / handrail.
My guess would be that 1/4" ply would give you all the strength you need, yet it will still give you the rigidity to make your finished panels smooth. I'm not sure, however, if they make 1/4" ply in an exterior grade.
If you use the Parkland product, glue it to the plywood while the panel is laying flat (with the proper, non-volitile glue) and before mounting the panel to the wall. To make sure you have the holes cut correctly, you could even "dry mount" the panel and mark the holes from the outside, through the port light cutouts. Then take the panel down again and cut the holes.
Good luck with your project. I'm sure it will look great!
Jack
