Marine plywood is supposed to not have any voids and it has the waterproof glue between veneers. The difference in price will determine how often you want to keep replacing it.
Yes you could cut the parts then epoxy them and do like capesams and paint with california porch paint, however, fir ply will check and it's best to also cover the wood with a skin of 6 or 9 ounce cloth also when epoxying. three coats of epoxy each side for waterproofing.
it will last a long time if you do it right the first time.
here is a place that has the marine ply alot cheaper than boulter plywood.
http://www.mainecoastlumber.com/
and they have Meranti which you don't have to use the fiberglass cloth since that wood won't check like fir does.