This paper presents a nonlinear controller based on a double integrator system for trajectory tracking of a nonlinear autonomous surface vessel. Using a fixed-point on the vessel body frame as position output, we devise a simplified controller with only two error states, as opposed to the four required in a more straightforward and naive backstepping approach. The resulting controller is dynamically simple and easy to implement, and does not require higher than second-order plant dynamics and reference trajectory derivatives. The proposed approach mitigates typical problems arising during backstepping such as noise amplification and the need for a highly accurate plant model. Experimental results with an instrumented autonomous surface vessel are presented to corroborate the performance and robustness of the proposed controller.