What is the main purpose of sound signals in navigation?

The main purpose of sound signals in navigation is to communicate a vessel's intentions. This is crucial in maintaining safety and preventing collisions at sea, especially in situations where visibility is limited due to fog, rain, or darkness. By using sound signals, a vessel can inform other vessels of its movements, such as whether it is altering course, approaching, or overtaking another vessel.

While other options address important aspects of navigation, they do not capture the primary function of sound signals. Alerting nearby vessels of presence, signaling distress, and indicating a change in course are all fundamental components of maritime communication, but they fall under the broader umbrella of communicating intentions. Therefore, sound signals serve as a vital tool for vessels to convey their actions and ensure coordinated movement in crowded or hazardous waters.