What is the meaning of five blasts on a ship's horn?

Five short horn blasts from another boat on the water usually means danger. It could mean they have doubt in the message they had received from you, whether they did not hear it or did not understand. It can also be used to signal that the boat driver is confused about who has the right away. It is a signal to alert another vessel that you are unsure of its intentions, or doubt whether you are taking enough action to avoid a collision.