This paper presents a force control strategy for a manipulator/vehicle system floating on the water. The control strategy reduces the number of actuators required for the force control by utilizing the restoring force/moment applied to the vehicle. We derive the relation between thruster outputs and the endpoint force/moment based on the kinematics and dynamics of the system taking the restoring force into account. Using this relation, we calculate the realizable force/moment at the endpoint. Simulation results illustrate the validity of the proposed algorithm.