Boston Harbor handles more cruise passengers than any New England port, yet its 180 square miles of sheltered water remain defined by the working piers, naval berths, and island fortifications that shaped three centuries of American maritime commerce. The harbor's 34 islands—27 protected within Boston Harbor Islands National Recreation Area since 1996—bear witness to colonial trade, Revolutionary War sieges, Civil War prisons, and Cold War radar stations. Fort Independence on Castle Island, rebuilt in granite between 1834 and 1851, stands where British cannons once commanded the Narrows; Deer Island's wastewater treatment plant, completed in 2000, reversed a century of pollution that had closed shellfish beds and beaches across the outer harbor. Boston boat tours & sightseeing depart from Long Wharf, constructed in 1710 and extended 1,586 feet into the harbor to accommodate tall ships carrying rum, molasses, and African slaves in the triangular trade. The wharf's granite warehouses, rebuilt after the 1872 Great Boston Fire, now anchor a waterfront transformed by the Central Artery's removal in 2007. USS Constitution, launched in 1797 and moored at Charlestown Navy Yard since 1897, remains the world's oldest commissioned warship afloat; her copper sheathing, rolled by Paul Revere's Canton foundry in 1803, protected live oak timbers harvested from Georgia sea islands. The harbor's commercial role persists—Conley Terminal handles 1.3 million TEUs annually, and the Black Falcon Cruise Terminal berthed 398,000 passengers in 2025. Harbor sightseeing routes pass the Boston Light on Little Brewster Island, first lit in 1716 and automated in 1998, making it the last staffed lighthouse in the United States. Stellwagen Bank National Marine Sanctuary, 25 miles east of Boston, sustains feeding populations of humpback, fin, and minke whales from April through October; the 842-square-mile sanctuary, designated in 1992, protects glacial-carved underwater plateaus where nutrient upwellings concentrate sand lance and herring. Sunset cruises trace the Inner Harbor's arc from the Seaport's glass towers—50 Liberty Wharf opened in 2011, One Seaport Square in 2017—past the Federal Reserve's aluminum-clad headquarters to the Zakim Bridge's cable stays, lit in programmable LEDs since 2008. The harbor's depth, dredged to 47 feet in 2016 to accommodate post-Panamax container ships, erased mudflats where 18th-century shipwrights launched brigs and schooners at low tide.