What is a common method to indicate distress at sea?

Sounding a ship's horn multiple times is a recognized method to indicate distress at sea. In maritime communication, the use of sound signals is an important way to convey messages, especially when visual communication is not possible. A series of short blasts on the horn serves as a universal alert to other vessels in the vicinity that assistance may be required. This method aligns with the International Regulations for Preventing Collisions at Sea (COLREGs), which emphasize the importance of sound signals in signaling distress.

The other methods listed are less effective as distress signals. For instance, raising a flag at half-mast is generally used for mourning or honoring someone who has passed, not specifically for indicating distress at sea. Firing flares is indeed a distress signal, but it is effective at any time of day, not just at night, and hence implies that limiting their use only to nighttime is misleading. Rapidly changing course, while it might indicate to others that something is wrong, does not provide a clear or standard signal of distress.