Experimental testing of an unmanned surface vehicle (USV) has been performed to evaluate the performance of two lowlevel controllers when displacement and drag properties are time varying and uncertain. TheUSV is a 4.3-m-long, 150-kg wave adaptive modular vessel (WAM-V) with an inflatable twin-hull configuration and waterjet propulsion. Open-loop maneuvering tests were conducted to characterize the dynamics of the vehicle. The hydrodynamic coefficients of the vehicle were determined through system identification of the maneuvering data and were used for simulations during control system development. The resulting controllers were experimentally field tested on-water. Variable mass and drag tests show that the vehicle is best controlled by a model reference adaptive backstepping speed and heading controller. The backstepping controller developed by Liao et al. (2010) is modified to account for an overprediction of necessary control action and motor saturation. It is shown that when an adaptive algorithm is implemented for the surge control subsystem of the modified backstepping controller, the effects of variable mass and drag are mitigated.