A boater sounds 5 short blast of air horn. What does it mean?

Five short blasts of an air horn typically indicate a warning or a signal of danger on the water. It can convey that the vessel is unsure of another boater's intentions or that it is not safe to proceed. This signal is part of the International Regulations for Preventing Collisions at Sea (COLREGs) and is used to promote safety and communication among vessels.