What's the difference between a ship and a boat?

According to the Oxford English Dictionary, a ship is:

a large sea-going vessel (opposed to a boat); especially (in modern times) a vessel having a bowsprit and three masts, each of which consists of a lower, top, and topgallant mast.
In Old English the word was used also for small craft

A boat is defined as:

(a) a small open vessel in which to traverse the surface of water, usually propelled by oars, though sometimes by a sail.

(b) extended to various vessels either smaller than, or in some way differing from, a ‘ship’; esp. small sailing vessels employed in fishing, or in carrying mails and packets, and small steamers. (Sometimes applied to large ocean steamers, though these are more properly ‘steam ships’.)

I’m not sure when these definitions were drawn up: some bits of the OED go back to the late nineteenth century.

So the OED seems to agree with Halcyon: a ship is usually pretty big; a boat is usually small.

Here are a couple stabs at defining the difference:

1. Among sailing vessels, the distinction between ships and boats is that a ship is a square-rigged craft with at least three masts, and a boat isn't. With regard to motorized craft, a ship is a large vessel intended for oceangoing or at least deep-water transport, and a boat is anything else.

Try this: ships have to be big enough to carry boats, and boats have to be small enough to be carried by ships.

2. distinction between a ship and a boat varies depending on regional definitions, but as a general rule, a boat can fit onto a ship, but a ship cannot fit onto a boat. A ship, in other words, is a very large ocean-going vessel, while a boat tends to be much smaller. Additionally, a ship usually is defined as having a displacement larger than 500 tons.

3. According to the Navy sailor who conducted our tour, "the difference between a ship and a boat is that you can put boats on a ship, but you can't put ships on a boat. But submarines are always boats." According to Chapman Piloting, Seamanship and Small Boat Handling (a great reference book), the dividing line of size between boats (small) and ships (big) is 65 feet. To further confuse things, the same reference states that a "yacht" is a "sail or power vessel used for recreation and pleasure, as opposed to work," but also adds that the term is usually not used for boats under 40'. I've also heard that it's a boat when you have to work on it but a yacht when you take your sweetheart out on it.

http://www.straightdope.com/columns/read/754/whats-the-difference-between-a-boat-and-a-ship
http://www.wisegeek.com/what-is-the-difference-between-a-ship-and-a-boat.htm
https://web.archive.org/web/20040815170235/http://windom.cybox.com/logs/1999/05.28.html

The ship/boat distinction varies from region to region. Basically, the rule of thumb is "a boat can fit onto a ship but a ship can't fit onto a boat." Also, a ship is a ocean-going vessel while a boat will tend to stick closer to smaller bodies of water. A ship is also classed as a vessel with a water displacement of at least 500 tons, while a boat would not displace that much water.