What does a continuous sound signal of a horn indicate?

A continuous sound signal of a horn is used to indicate that a vessel is in distress. This signal is part of the communication protocol for mariners, as distress signals are crucial for alerting nearby vessels of an emergency situation. The continuous horn sound can be perceived clearly over distances, which makes it an effective way to draw attention to a boat needing immediate help.

The other options do not correlate with the use of a continuous sound signal. For instance, safety drills typically utilize different sound patterns, such as a series of short blasts, to avoid confusion. Similarly, sounds associated with approaching a harbor or preparing to depart are governed by specific signaling rules that do not include continuous horn blasts, which are exclusively reserved for distress situations. This ensures clarity in communication at sea, where misunderstanding can lead to serious consequences.