I have a older river craft boat. It's justice a little over 4 feet wide. It's almost identical to the Randall craft, Collins and Kennedy craft boats built up here in the Florida panhandle. I don't have a good source of marine grade plywood. I've done some research and found that good exterior grade is close to the same quality. I will be encapsulating it in fiberglass and epoxy resin. If the boat lasts another 10 years it more than I could ask for. Any reason not to use the exterior grade for this ?