In spite of these well-defined rules, often the thin line of difference between a boat and ship is often overlooked, which eventually leads to confusion. For example, a submarine is not an underwater ship, but is classified a boat. This is because when submarines were first made they were controlled and moved using a mother ship. Even today large ships are used to lift and tow submarines, though some cannot be carried onboard. Thus submarines are still termed as boats in spite of the fact that the lengths of few of them are equal to those of the largest ships.
Even some of the leisure vessels such as kayaks, catamarans, yachts, ferries, tugs, and canoes are termed as boats. Also, large commercial fishing vessels, which can be easily termed ships, are called boats. When one thinks about ships, they imagine large sea going vessels such as cruise ships, container and tanker vessels, aircraft carriers, etc. Thus traditionally, ships are vessels with large cargo and crew carrying capacity, along with sophisticated technology on board.
Thus, finding a difference between a ship and a boat can be a daunting task, especially when the traditional features of both of them overlap. Moreover, the term by which a particular vessel is known also depends on the region it belongs to. But throughout the world, a ship or a boat is mostly known by its size and the function that it performs.
Image Credits : Boat at reedboat website (http://www.reedboat.org/The%20Boat/testBoat.jpg)