Try Mr. Plywood in SE Portland. It's off of 82nd street. I cant remember the street name it's on, but I think it's up around the Burnside area. They have (or had)a good selection. They will tell you that the only difference between Marine and a quality exterior grade plywood is the absence of voids within the veneers on the marine grade. I believe they use the same glue. The absence of voids will eliminate weak spots in the hull (hulls curve) and transom. If the areas you will be rebuilding are above the waterline, exterior grade should probably be OK. Just don't be cheap. Buy good plywood. Paint it or seal it well. Of course, I'm sure somebody will disagree with this, but I have been down this road many times and that's my two cents worth.