Which characteristic primarily defines the operational function of a WIG craft?

The operational function of a WIG (Wing In Ground Effect) craft is primarily defined by its ability to fly above the water's surface utilizing surface-effect action. This unique characteristic allows the craft to generate lift through the interaction of its wings with the surface of the water, effectively reducing drag and allowing for higher speeds with improved fuel efficiency.

Unlike traditional aircraft, which rely solely on altitude for flight, WIG crafts operate efficiently close to the water, leveraging the aerodynamic benefits of ground effect to enhance their performance. This operational mode enables WIG crafts to maneuver effectively over water, making them suitable for various applications including transport and search-and-rescue missions.

In contrast, operating entirely underwater would define a submersible, while a craft that travels on land and water would belong to the category of amphibious vehicles. Lastly, a sailing vessel is characterized by its reliance on wind as its primary means of propulsion, which is fundamentally different from the surface-effect operational mode of WIG crafts.