Marine Grade plywood is a sturdier, higher-quality material made of Western Larch or Douglas-fir that is not treated with chemicals. This type of wood’s thin sheets of veneer (plies) are usually set up perpendicularly as part of a process called cross-lamination that helps strengthen the material. It is a good-quality, hardwood plywood made with waterproof glue, but since it’s not treated with chemicals it is not rot-resistant.