Marine-grade plywood is made of thin layers of high-quality hardwood held together with waterproof glue. Although it is not waterproof wood, it is water resistant, lightweight, strong, and virtually defect-free. It can last 15 to 25 years, especially when coated with a protective finish. Marine-grade plywood is used for wood exposed to a lot of water, including boats and nautical uses, outdoor garden furniture, decks, porches, and wood structures.