A watercraft is a vehicle designed to float on and move across (or through) water for pleasure, physical exercise (in the case of many small boats), transporting people and/or goods, or military missions.

The common need for bouyancy unites all watercraft, and makes each one's hull a dominant aspect of its construction, maintenance, and appearance.

Most watercraft would be described as either ships or boats; although nearly all ships are larger than nearly all boats, the distinction between those two categories is not one of size per se.

• A rule of thumb says "a boat can fit on a ship, but a ship can't fit on a boat", and a ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts.
• Often local law and regulation will define the exact size (or the number of masts) that distinguishes a ship from boats.
• Traditionally submarines were called "boats", perhaps reflecting their cramped conditions: small size reduces the need for power, and thus the need to surface or snorkel for a supply of the air that running diesel engines requires; in contrast, nuclear-powered submarines' reactors supply abundant power without consuming air, and such craft are large, much roomier, and classed as ships.

Another definition says a ship is any floating craft that transports cargo for the purpose of earning revenue; in that context, passenger ships transport "supercargo", another name for passengers or persons not working on board. However, neither fishing boats nor ferries are considered ships, though both carry cargo (their catch of the day or passengers) (and for that matter lifeboats).

English seldom uses the term "watercraft" to describe any specific indvidual object (and probably then only as a affectation): rather the term serves to unify the category that ranges from small boats to the largest ships, and also includes the diverse watercraft for which some term even more specific than ship or boat (e.g., canoe, kayak, raft, barge, jet ski) comes to mind first. (Some of these would even be considered at best questionable as examples of boats.)

Examples of watercraft include:

• boat
• ship
• canoe
• raft
• kayak
• yacht
• dhow
• junk
• narrowboat
• coracle
• gondola
• catamaran
• catboat
• personal watercraft
• racing shell
• amphibious landing craft
• submarine