Ditto what McGillicuddy said.
Marine fir plywood is similar to a good quality A/C Fir plywood,but in most cases has more Plys, and there are no voids in any of them.Regular plywoods used in the construction industry will have voids in the plys that can allow water intrusion.
You will find that the Marine plwood will also be a more rigid and stronger plywood than the regular plywood.
Personally I'd spend the extra few bucks for the marine product.The last time I bought Marine plywood was about 3 years ago,and at that time I paid $78.00 for a 4x8 sheet of 3/4".
If you have any specialty plywood houses in your area they will sometimes sell it to you directly.If not any good independent lumber yard can order it for you.
Here's a picture of two pieces of 3/4" marine fir glued together.The dark line(the black dashes) in the center is so that you can see the epoxy glue joint where I glued them together for my transom.
You can also see there are no voids,and you can see that there are more plys than regular plywood.
