This paper presents a control scheme for obtaining high maneuverability of underwater robot manipulators mounted on a remotely operated vehicle (ROV). The motions of an underwater manipulator can affect the attitude and position of the ROV which should remain stationary in seabed operation. To compensate for the dynamic effect of the underwater manipulator on the ROV, force-torque (F/T) information between the manipulator and the vehicle is used to regulate the states of the ROV. When an F/T sensor is practically unavailable, a disturbance observer can fill the role of the F/T sensor This paper proposes a disturbance observer for estimating the interaction forces between the ROV and the manipulator A two-link manipulator mounted on an ROV is considered and numerical simulations are performed to demonstrate the improvement on the maneuverability of the proposed controller.