A wave energy converter (1) has: a buoyant structure (2) which, in use, floats on water; a generator (18); a generator drive mechanism (38) on board the buoyant structure (2), the generator drive mechanism (38) having an rotational input drive shaft (20) and a rotational output drive shaft (36); a drive member (22) operably connected to the input drive shaft (20), the drive member (22) being moveable back and forth between a first position and a second position; a biasing arrangement (23, 26) for example a buoyant block acting on the drive member; and, a submerged element 4, 4′ which, in use, moves below the surface of the water out of phase with the buoyant structure (2), the drive member (22) being attached by a tether (28) to the submerged element (4). In use, when the buoyant structure (2) encounters a wave crest, the spacing between the buoyant structure (2) and the submerged element (4, 4′) increases and the drive member (22) is pulled towards the second position by the tether (28), and, when the buoyant structure (2) encounters a wave trough, the spacing between the buoyant structure (2) and the submerged element (4, 4′) decreases and the drive member (22) is urged towards the first position by the biasing arrangement (23, 26). The back and forth movement of the drive member (22) between the first and second positions causes the input drive shaft (20) to rotate and, thereby, causes the output drive shaft (36) to rotate. The submerged element (4, 4′) is preferably a heave plate. The invention also comprises a heave plate for a submerged, partly submerged or floating structure.