German military style whistle is so loud that you can hear it over a mile away. It's small, lightweight, imported.
One of the oldest pieces of sailing equipment, the Boatswain's whistle (Boatswain's pipe, Bosun's pipe or Boatswain's flute), was used in ancient Greece and Rome to indicate the stroke for galley slaves and during the Crusades to call English cross bowmen on deck for attack. Its shrill sound could be heard on deck and so it was later used to signal the boarding of officials. This signaling device became a badge of office and honor in the British and American Navys and is still used today.