NOVELTY - A subsurface reaction body (1) is selectively filled with air to a maximum buoyancy configuration or with water to a minimum buoyancy configuration. The reaction body also has at least one further preset buoyancy configuration between minimum and maximum buoyancy configurations, and includes an array of chambers (12) made airtight when filled with air but not watertight when filled with water. The chambers can also inhibit free flow of water through the body.    USE - Subsurface reaction body for use in apparatus for converting wave motion in body of water, such as sea, into useful energy (claimed).    ADVANTAGE - The subsurface reaction body is suspended from the buoyant body at depth sufficient to ensure that the reaction body is below the influence of waves on the sea surface, such that movement of the buoyant body caused by waves results in relative motion between both bodies which can be exploited by the energy converter. The reaction body has a large surface area perpendicular to the direction of heave force, providing resistance to the buoyant body movement by large drag and added mass. The converting apparatus including the reaction body can sit sufficiently high in the water so that all connections to the mooring and power lies can be clear of water and be easily accessible. The apparatus can also create its own stable service platform with all serviceable components clear of water to permit easy access for maintenance. Expense of fabricating sealed watertight and or airtight chambers can be avoided, allowing the reaction body to be made at a lower cost. The reaction body can be made from a framework providing a superior structure when the apparatus is used in transferring large loads. The bulkheads within the framework can be made of low cost material such as corrugated steel, fiberglass, or high strength fabric. Sufficient submerged weight can be given to the reaction body by using low cost porous concrete as ballast.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an apparatus for converting wave motion in body of water into useful energy.    DESCRIPTION OF DRAWING(S) - The drawing shows the perspective view of an apparatus for converting wave energy into useful energy equipped with a reaction body.    Subsurface reaction body (1)    Float (2)    Connecting line (3)    Spring-loaded energy converter (4)    Chambers (12)