TotalBoat FixWood can be used below the waterline, however, it is extremely important to first establish if it is the right product to use. FixWood should never be used as a seam compound, or to fill rotted wood below the waterline on wooden substrates that can come in contact with the water. Wood that comes in contact with water expands and contracts, and there are products specifically designed for caulking and seams. Rotten wood, or missing sections of wood below the waterline, should never be filled with FixWood, as it is just a filler. It is not designed to absorb dramatic expansion and contraction of the wood around it, or other stresses that wood planking or plywood would otherwise handle comfortably. The nature of wood being saturated with water when below the waterline also means that it may reject the epoxy if it flexes, expands, or contracts too much. In most cases, FixWood can be used for small fairing applications where there isn't a lot of flexing or induced stresses.