Plasma wave resonances are studied in a two-dimensional plasma of positive ions confined just below the surface of liquid helium. The resonances are observed over the range from 14 mK to 400 mK at ion densities up to 1×108/cm2. The resonant frequency is determined by the ion effective mass, and the line width measures the scattering from the excitations of the helium surface. Strong coupling of the ions to the ripplons is evidenced by a rapid increase of the line width as the perpendicular electric field is increased to the surface. Comparison with the theory of Shikin and Monarkha [1] is presented. The Wigner crystallization transition has been searched for, but no change in the plasma wave characteristics has been observed over the range 20<Γ<2000, where Γ is the ratio of Coulomb energy to kinetic energy.