The title of the world's largest boat is one that obviously changes quite frequently. New boats are constantly being built, and boat makers are constantly pushing the envelope of size. For the time being, though, the largest seafaring vessel by size ever built is the MV Mont (formerly the Knock Nevis). At a massive 458.45 meters (that's 1504.10 feet), the boat is over a quarter mile long; it is 68.8 meters (225.72 feet) wide. The MV Mont's gross tonnage is 260,941 tons, and its capacity is 564,763 deadweight tonnage. This is the highest deadweight tonnage of any boat in history. The boat was built in Japan in 1979 by Sumitomo Heavy Industries.
It was so large, with a 24.6 meter (80.7 feet) draught, that it couldn't pass through many of the world's major shipping passages: the Nevis could not sail through the Suez Canal, the English Channel, or the Panama canal. The ship was capable of a sixteen knot speed, powered by steam propulsion. The ship has been renamed the MV Mont by its current owners, who have deliberately beached the ship and intend to junk it.