A marine heave compensating device comprises a hydraulic piston and cylinder unit (15, 16) with its cylinder (15) secured to a support (10,11). The piston is connected via a piston rod (16) to a cross-head (17) on which a first set of pulleys (18) are rotatably mounted. Chains (19) secured at one end to the support (10,11) are entrained around pulleys (18) and support a yoke (20) on which is mounted a crown block (21). A cable (22) is reeved around crown block (21) and around a travelling block (23) which carries a tool carrier (24). The two ends of cable (22) are reeved around two second pulleys (26A,26B) rotatably mounted on the cross-head (17). End (22A) of cable (22) is connected to a draw-works and end (228) of cable (22) is connected to a tie-down.