When it comes to basic terminology, there isn't much of a difference between a boat and a ship (though the military rule of thumb is you can put a boat on a ship, but you can't put a ship on a boat). However -- and this is paraphrased from "Chapman Piloting: Seamanship & Boat Handling" (63rd Edition), the mariner's bible for all things maritime -- though there is no official difference, the line is generally drawn at 20 meters (or about 60 feet), at which point a "boat" becomes a "ship," though neither designation is incorrect for any length.

The cruise industry agrees with the 20 meter/60 feet dividing line. River cruising generally involves riverboats (exceptions to the "rule" include 60-plus-feet options like Viking River Cruises' Longships and Avalon Waterways' Suite Ships). Expedition lines have adopted the term "ship." Meanwhile, larger oceangoing cruise ships have been divided into four categories, based on size: mega-ships, big ships, midsize ships and small ships.

#inCruises

Bottom line: Neither "ship" nor "boat" is technically wrong, but you'll sound a whole lot savvier if you follow proper cruise industry etiquette.