Erik_Kowal wrote: ↑Wed Apr 15, 2020 11:00 pm
Ships are large vessels defined primarily in terms of their size, and secondarily by the fact that they are mostly seagoing vessels rather than freshwater craft.
Unless they are submarines in the Royal Navy, in which case they are always "boats".
While Erik's explanation generally holds true, many people use "boat" as a generic term for vessels of all sizes. But the term "ship" is virtually always reserved for large vessels designed to carry large amounts of passengers or cargo. That said, as Erik intimated, passenger and cargo vessels used on rivers and smaller lakes are generally called "boats", even if they are larger than some of their seagoing cousins. On larger lakes, such as the Great Lakes of the USA, or even Lake Constance in Europe, large vessels will be called ships.
But the principle is relatively simple: Wherever you decide to put the boundary between a "ship" and a "boat", about half of all native speakers will disagree with you!