Please note that the PICTURE UPLOAD function is not working. The iBoats Forum Moderators are working with the system administration folks at the parent company to get the issue resolved. This happened about a year ago and here we are again. In the meantime you can post photos as links to a picture hosting site of your choice. We are sorry about the inconvenience and will advise when the issue is resolved. 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.