The surveillance task of the French Navy's signal stations involves building and maintaining a Situation Awareness [1], enabling the detection and treatment of any abnormal or risky situation. In this context, the visual watch carried out by the lookouts is of vital importance to the reliability of the monitoring system. The activity deployed, observed in the field and through the film of day shifts, shows a homogeneity in practices carried out between lookouts with varied profiles. The verbalization of these actions in interviews, however, reveals a wide diversity of intentions and explanations underlying these observables. The notion of professional vision [8] thus appears to be a suitable tool for a descriptive analysis of practices within a profession, but not very well suited itself to an in-depth understanding of the activity from an ergonomic point of view. It is with this in mind that we propose the notion of community practices, to capture the diversity of activities deployed by members of a profession.