What's the difference between a boat and a ship?
Commonly, a ship is a large craft, a boat a small one. According to the U.S. Navy, you can put a boat on a ship, but you can't put a ship on a boat. A submarine is referred to as a boat, because of its beginnings as a very small craft. Some large yachts SHOULD be called ships, but often are not.
Size is the most commonly stated difference between a boat and a ship. Typically a ship is a larger, ocean going sea vessel. A boat is a smaller water vessel. Some use the reminder that a ship can carry a boat but a boat can never carry a ship.