In this paper we develop an event-triggered mechanism to track a reference trajectory using an underactuated autonomous surface vessel. To achieve this goal, we design a backstepping control law that guarantees the total system error is uniformly ultimately bounded. Secondly, building upon a diffeomorphism relating the errors to the system state, we propose an event-triggered control mechanism for the closed-loop system that implements the sample-and-hold of the state signal, and renders a compact neighborhood of the zero tracking error set globally asymptotically stable for the closed-loop system. Simulation results are presented to validate and illustrate the performance of the proposed solution which, in addition to pose as an intrinsic digital control implementation with stability and performance guarantees, is also efficient, as shown, in trimming down controller updates whenever appropriate.