A hydraulically-powered subsea tool system has a tool (26) and a tool power unit (12). The tool (26) communicates with a first connector element (30) on a hydraulic drive circuit (82). The tool power unit (12) has a hydraulic supply circuit (88) communicating with a second connector element (20). The connector elements are wet-mated with each other to connect the circuits for powering the tool. The circuits are pressure-compensated by respective compensators (136,90). When the circuits are connected, a valve (140) is operable to transfer pressure compensation of one of the circuits from the compensator (136) of that circuit to the compensator (90) of the other circuit. The valve (140) suitably transfers pressure compensation of the drive circuit from a drive circuit compensator to a supply circuit compensator.