A passive heave compensator comprising: a main hydraulic cylinder, including a moveable piston having a piston rod extendible through the main hydraulic cylinder and a piston head to divide the main hydraulic cylinder between a gas phase above the piston head, and oil phase below the piston head; an upper connection point associated with the main hydraulic cylinder and a lower connection point associated with the piston rod; and an accumulator having a moveable separator to divide the accumulator between a gas phase above the separator, and an oil phase below the separator and being in communication with the oil phase in the main hydraulic cylinder; characterized in that an oil phase includes a magnetorheological substance, and that the passive heave compensator includes one or more electromagnetic controllers. In this way, operation of the one or more electromagnetic controllers can magnetise the magnetorheological substance to vary the viscosity of the oil phase, thus changing the degree of damping that can be provided by the passive heave compensator.