A stand-up paddle board is formed by a core surrounded by a skin. The core is manufactured by creating an upper core member mold and a lower core member mold, corresponding to the top and bottom of the core. The upper core member and lower core member includes a concave surface filled by a lattice-shaped matrix. The core members and the matrices are molded to be complimentary in shape, such that the surface of each matrix and the surrounding upper and lower core members adjoin each other and are connected by an adhesive to create an air-tight core having air-tight matrix chambers. The completed core is then surrounded by a conventional paddle board skin.