Experiments on magnetoplasma waves in two-dimensional pools of ions trapped below the surface of superfluid 4He have been used to obtain values of the cyclotron effective masses of these ions down to a temperature of about 20 mK. The cyclotron masses are found not to depend on the depth at which the ions are trapped below the surface, in contrast to the apparent behaviour of the effective mass obtained from the plasma resonant frequency in zero magnetic field. For reasons that are discussed, the latter masses are thought now to be unreliable. No evidence has been found for either crystallization of the ion pools or the formation of ripplonic polarons.