It depends on the type of wood you are using and what kind of paint will be applied. Applying two coats of exterior paint to the plywood is a good idea for long-term use. The first coat should be a primer, which will help seal any knots or imperfections that could cause later problems. A good primer for exterior use is one made for use on wood. It would help if you made the second coat from marine-grade paint, which will protect the wood from moisture and saltwater. It is a good idea to apply a layer of varnish or polyurethane over the top of the paint to ensure that it lasts as long as possible. It is important to note that the type of wood you use will determine what kind of paint you should apply. There are different types of plywood, and the paint you use should be compatible with the kind you are using. For example, if your boat is made from cedar or redwood, then it is best to use paint that is made for cedar or redwood. If you are using plywood, you must check the type and ensure that it will not expand too much in the water. If you use paint incompatible with your plywood, it may crack or peel off easily.