The Bosun's, or boatswain's Whistle, is one of the oldest pieces of nautical equipment. It is the 'modern day' descendant of the flutes used by the Ancient Greeks and Romans to convey orders to the oarsmen and galley slaves. The first use of the whistle in the English Navy was during the crusades of the 13th century
The whistle is named after the Bosun -- a warrant or petty officer in charge of a ship's rigging, anchors, cables, and deck crew -- Bosun also being one of the oldest titles in seafaring history. The whistle was the Bosun's badge of office, and was often worn around his neck by a chain, and was used to "pipe" his to orders to the crew ("piping" is the naval method of passing orders).
In the days of sailing ships, verbal commands were often misunderstood -- or not heard at all above the the howling winds and the roaring seas. However, orders issued by the shrill tone of the Bosun's pipe could be heard above even the worst of gales. Today, the Bosun's whistle is still used to pipe the Captain or special visitors on board.