A buoyant
actuator (10) for use in apparatus (11) for harnessing ocean wave energy and for converting the harnessed energy to high-pressure
seawater. The buoyant
actuator (10) comprises a body (21) defining a chamber (23) having a pliant outer
skin (27). The chamber (23) is adapted to contain matter and a hydrodynamic property of the body (21) is selectively variable by varying the matter within the chamber (23). The variation to the hydrodynamic property may comprise a variation to the
buoyancy (either positively or negatively) or a variation to the
response area (such as the volume or shape) of the body (21), as well as a combination thereof. The variation to the matter may comprise addition of matter to, or extraction of matter from, the chamber (23). The matter may comprise a
solid, liquid or gas, as well as any combination thereof. In the arrangement shown, the matter comprises foam spheres (53). The outer
skin (27) is drawn into a taut condition by the outward pressure of the foam spheres (53) inside, causing the
actuator to assume its design shape. The volume occupied by the foam spheres (53) is in total still less than the total enclosed volume of the chamber (23) and there are interstitial regions (55) around each sphere (53). The interstitial regions (55) may be filled with fluid to adjust the
buoyancy.