Abstract: A subsurface reaction body arranged for use in apparatus for converting wave motion in a body of water to useful energy is provided. The subsurface reaction body is arranged to be selectively filled with air to a maximum buoyancy configuration and to be filled with water to a minimum buoyancy configuration. The subsurface reaction body has an arrangement of chambers therein to inhibit free flow of water through the reaction body and to define at least one further preset buoyancy configuration between the respective maximum and minimum configuration.