Nantucket Light Ship/LV-112

This stalwart vessel used to protect ships passing through the treacherous Nantucket Shoals 100 miles off the Massachusetts coast.  It's a floating lighthouse, complete with fog horn and beacon.  After weathering hurricanes, fog, blizzards, and storms for 39 years, she was decommissioned in 1975, and is now a National Historic Landmark, permanently moored in Boston Harbor.  You can read a fascinating article about this ship here.