http://www.se-technology.com/wig/index.php

 If you build a hovercraft, you induce drag, which increases resistance to the water, which decreases speed. 

 The ultimate low-drag marine craft would be a boat without water contact, a hovercraft without a fan: a WIG boat. A WIG boat is a boat with wings that cruises just above the water surface, it is floating on a cushion of relatively high-pressure air between its wing and the water surface. This cushion is created by the aerodynamic interaction between the wing and the surface, called ground effect. This is where a WIG boat is different from an aircraft, it cannot operate without ground effect, so its operating height is limited relative to its size.

 WIG is an abbreviation of Wing In Ground-effect. A WIG boat can be seen as a crossover between a hovercraft and an aircraft. It flies just above the surface, usually the water surface, therefore others use the term WISE or WISES (Wing In Surface Effect Ship). The Russians use the word Ekranoplan (Ekran = screen, plan = plane), which is also commonly used in other languages nowadays. Some other terms are commonly used in literature.

A hovercraft is floating on a cushion of air that is created by a fan that blows in a cavity under its hull. The cavity is bounded by so called skirts, rubber balloons that restrict air leakage and more or less seal the cavity. The air cushion reduces the friction drag of the hovercraft with the water which would make it a very efficient vehicle if is wasn't for the fan that creates the cushion. A WIG craft also sits on a cushion of air, but this cushion is created by aerodynamics rather than by an engine. This means that it only exists when the WIG craft has sufficient forward speed. This is called a dynamic air cushion as opposed to the hovercrafts static air cushion. You can compare this to the relation aircraft-helicopter, the aircrafts wings generate lift because of their forward speed, whereas the helicopters rotor has work continuously for generating lift. A more indepth explanation of ground effect aerodynamics can be found in the aerodynamics section.

 A WIG boat is especially designed to take advantage of the benefits of ground effect. Therefore it will always fly close to the surface. Although it is called ground effect, most WIG vehicles only operate over water, but some are amphibious. Some WIG vehicles have the ability to fly without ground effect as well, but inefficiently as compared to aircraft.