This paper is devoted to the problem of non-linear robust control design for Underwater Vehicle/Manipulator (UVM) systems composed of a free-floating platform equipped with a robot manipulator. The different bandwidth characteristics of the composite vehicle/arm dynamics are used as a base's for the control design via singular perturbation theory. The main contribution of this paper is that a robust nonlinear control is only required in the slow-subsystem (vehicle dynamics). Thus, This control is only used to compensate the coupling effects from the arm to the basis. The paper also presents a comparative study between simple PD, a partial linearized control and the nonlinear robust feedback..