GOOD NEWS. . . . The PICTURE UPLOAD FUNCTION IS WORKING !!! The tech folks at the parent company have been able to remedy the issue. You can upload pictures once again. Thank you for your patience.
The advantage of marine grade plywood is that it is solid core. Sure, ext will work but you better also figure in the cost of some real good caulking to fill in the holes or the water intrusion will later require you to do it over again.