This paper is devoted to the event-triggered course tracking control for a class of surface vessels (SVs). Notably, sampling of control input is considered under the coexistence of output constraint and unknown control direction, which is often overlooked in most related works. Other works often consider the sampling but ignore the output constraint. This leads to the inapplicability of traditional schemes on this topic. For this, two state transformations are first introduced for system output and reference signal, under which the boundedness of the new states implies the satisfaction of the output constraint. Then, two pivotal events are designed based on newly defined states, which facilitate the sampling mechanism of the controller and the updating mechanism of the controller parameters. By a skillful combination of the above two mechanisms, a switching event-triggered controller is explicitly designed, which guarantees that all the states of the resulting closed-loop system are bounded, while the system output practically tracks the reference signal, along with the satisfaction of the output constraint and the exclusion of the Zeno phenomenon. Finally, a simulation example is given to validate the effectiveness of the proposed theoretical results.