If you feel a sponginess underfoot that didn’t used to be present on the floor of your boat, chances are it’s an area that is slowly decaying and turning from solid plywood to a pulpy paste. In all likelihood if you don’t stop it, the decay will continue. An easy patch job sometimes can turn into a total floor replacement project. Either way, you will have to replace the decayed plywood with new material.
On regular plywood, only the exterior lamination is guaranteed to be a solid sheet. The laminations inside may contain voids where knots were present in the log or where cracks occurred during the lamination process. These minor imperfections cause few problems for users in many applications. One application which can be problematic with regular plywood is when it’s being used to form a boat hull. The voids can make weak spots or allow water to infiltrate into the core of the plywood. Marine-grade plywood contains no voids. Each laminate layer is a solid piece of wood.
Wood can be put in special chambers that use high pressure to infuse special chemicals into the wood to make it highly resistant to decay. Lumber treated in that fashion is used in areas where it will be in contact with the ground, in water or in areas where it will be prone to frequent wettings. The same treatment process can be used on ordinary plywood, increasing its ability to stay strong in uses that would quickly deteriorate untreated wood.
It costs more to produce marine-grade plywood since it uses higher quality wood and the manufacturing process is demanding. The demand for marine plywood is much less than for conventional or pressure-treated plywood so makers can’t increase profits through volume sales. Though the cost of the process to pressure treat regular plywood adds to its price, the demand for this product is much higher. Expect to pay more for marine-grade plywood than for pressure-treated plywood.
Which to Use
There is no structural advantage to using marine-grade plywood in a boat’s floor. The solid laminate layers inside won’t make the floor significantly stronger, it will cost more initially and marine plywood is not resistant to decay. The woodworking skills needed to do the repair and installation are identical. You’ll likely be better served by using pressure-treated plywood. However, if appearance is important or if any edges of the plywood will be exposed, you might prefer the higher quality veneers and construction of marine plywood. For example, you can use a router or rotary tool to mill a smooth rounded edge on marine plywood without the chance of exposing open pockets within the plywood's layers.