The adsorption, mobility, and self-association of naphthalene (NPH) and 1-methylnaphthalene (1MN), two of the simplest polycyclic aromatic hydrocarbons (PAHs), at the surface of liquid water at 289 K were investigated using classical molecular dynamics (MD) simulations and free energy profile calculations across the water-vapor interface. Both NPH and 1MN, which exhibit a strong preference to be adsorbed at the water-vapor interface, are found to readily self-associate at the water surface, adopting mostly configurations with distinctly nonparallel arrangement of the two monomers. The additional methyl group of 1MN represents only a minor perturbation in terms of the hydration properties, interfacial orientation, and self-association with respect to NPH. Implications of the observed fluorescence spectroscopy of NPH and 1MN in aqueous interfacial environment are discussed.