A ski structure for use with a boat hull to support the hull above the water when the boat is underway, the ski structure comprising an air tight housing in the form of an inverted U-shaped shell and a floor which forms a main ski surface and which extends between opposite sides of the U-shaped shell. The ski structure also includes a pair of assist skies or glides positioned lower than the main ski surface which extend substantially the length of the shell and which are adapted to plane on the surface of the water.