A heave compensating system for a marine vessel, the system comprising: a hydraulic actuator arranged for connection between the vessel and a load to be suspended from the vessel, the actuator being hydraulically actuable to vary the distance between the load and the vessel in response to heaving motion of the vessel; the hydraulic actuator being fluidly connected to a first hydraulic machine for actuation by the first hydraulic machine; the system further comprising a second hydraulic machine in fluid connection with a hydraulic accumulator; an electric motor; and a controller arranged to control hydraulic movement of the first and second hydraulic machines and power to said electric motor in response to one or more signals representative of a wave-induced heave movement of the vessel and/or a wave induced force applied to the load; characterised by the first and second hydraulic machines being mechanically connected to one another and also both being mechanically connected to said electric motor; wherein the first and second hydraulic machines are arranged to alternate between operating in a first drive phase and a second drive phase as the wave-induced heave movement of the vessel and/or the wave induced force applied to the load alternates between an upwards movement and a downwards movement, wherein: in the first drive phase, the first hydraulic machine operates as a pump and the second hydraulic machine operates as a motor, and in the second drive phase, the first hydraulic machine operates as a motor and the second hydraulic machine operates as a pump. a hydraulic actuator arranged for connection between the vessel and a load to be suspended from the vessel, the actuator being hydraulically actuable to vary the distance between the load and the vessel in response to heaving motion of the vessel; the hydraulic actuator being fluidly connected to a first hydraulic machine for actuation by the first hydraulic machine; the system further comprising a second hydraulic machine in fluid connection with a hydraulic accumulator; an electric motor; and a controller arranged to control hydraulic movement of the first and second hydraulic machines and power to said electric motor in response to one or more signals representative of a wave-induced heave movement of the vessel and/or a wave induced force applied to the load; characterised by the first and second hydraulic machines being mechanically connected to one another and also both being mechanically connected to said electric motor; wherein the first and second hydraulic machines are arranged to alternate between operating in a first drive phase and a second drive phase as the wave-induced heave movement of the vessel and/or the wave induced force applied to the load alternates between an upwards movement and a downwards movement, wherein: in the first drive phase, the first hydraulic machine operates as a pump and the second hydraulic machine operates as a motor, and in the second drive phase, the first hydraulic machine operates as a motor and the second hydraulic machine operates as a pump. in the first drive phase, the first hydraulic machine operates as a pump and the second hydraulic machine operates as a motor, and in the second drive phase, the first hydraulic machine operates as a motor and the second hydraulic machine operates as a pump.