From the linked Wikipedia article: "For reasons of naval tradition, submarines are usually referred to as 'boats' rather than 'ships', regardless of their size

From the linked Wikipedia article: "For reasons of naval tradition, submarines are usually referred to as 'boats' rather than 'ships', regardless of their size and shape." Here's a more verbose explanation which comes down to the same thing: http://www.subvetpaul.com/WhyBoats.htm http://www.subvetpaul.com/WhyBoats.htm