We investigate a three-wave decay interaction involving a surface wave in which a bulk electromagnetic plasma wave decays into another bulk electromagnetic plasma wave and a surface polariton. It is shown that a p-polarized light wave transmitted from vacuum into a plasma at a particular angle of incidence can propagate as a bulk electromagnetic plasma wave without attenuation. The nonlinear boundary conditions for the surface wave are formulated in terms of the surface charge and the volume current, taking full account of the rippling of the free boundary. The coupled-mode equations are derived and solved in the parametric approximation to obtain the threshold and the growth rate. We assess the relative strength of the kinematic and the dynamic nonlinearities in the parametric interaction.