This paper presents an adaptive sliding mode control structure of underactuated unmanned surface vessel systems under parametric uncertainty. The primary motivation in this research is to compensate for disturbances related to the added hydrodynamic forces and moment in the nonlinear control of a three degree-of-freedom marine vessel. The novelty of this work is the tracking robustness and the compensation for uncertainties common to surface vessels. The first work is to divide the dynamic model of the system into the ship rigid-body terms and added terms induced by hydrodynamics. A sliding-mode controller is designed to force the error trajectory into the sliding surface, which produces a robust tracking result in a finite time. For the parametric uncertainties in the dynamic model, an adaptive controller is designed to compensate using a projection-based adaptation law. After combining these two control schemes, a closed-loop controller designed by a Lyapunov-based control approach over feedback linearization is appropriately designed to yield the nonlinear tracking system bounded in the presence of uncertainties. The mathematical proof shows that a stable tracking result in the sense of Lyapunov-type stability is achieved. Numerical simulation results are shown to demonstrate the validity of these proposed controllers.