This paper describes an approach to calculate the radiation efficiency of infinite plates subject to a point load for any frequency. The results obtained by this approach are valid for the radiation efficiency of point-driven finite plates in water with real world material properties for frequencies near and above coincidence. This approach can be used to guide experimental measurements and numerical calculations of radiation efficiency as the excitation frequency is increased for finite plates in water. The radiation efficiency calculation for an infinite plate for all frequencies is derived for the first time. The limitation of thin plate theory in this application is also assessed by comparing it with the thick plate theory used in this work. An important distinct behavior above the coincidence frequency in a heavy fluid is revealed through the calculation of the radiation efficiency and explained by the dispersion relation of an infinite plate fully coupled with a fluid. In contrast to the case of light fluid (e.g., air), the radiation efficiency in a heavy fluid (e.g., water) does not approach unity in the neighbourhood of the coincidence frequency but at a significantly higher frequency.