In days of yore, there were only yachts. Until there came superyachts. Twice as large as their predecessors, with midocean necessities such as spiral staircases, split-level saloons and flybridge hot tubs, these floating mansions did more than eclipse conventional yachts -- they made them appear impish, toylike. And then, along came megayachts. These Poseidons, in good truth, possessed not only grandeur (nobody dared describe a yacht less than 150 feet as a "mega"), but touches of lavishness, such as crystal elevators and goatskin walls. They even came with gyroscopically controlled swimming pools -- to keep the water smooth in the...