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 buoyancy 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.