Billionaires are demanding that waterfalls be installed in their superyachts -- and they have a really good reason for it

The ultimate rite of passage for any billionaire is to get a superyacht -- a floating palace in which the megarich can live in luxury on the sea. These boats have cutting-edge technology, including fixed-pitch propellers combined with a Rolls-Royce central booster jet that lets them travel at the speed of a car. The interiors put five-star hotels to shame, too. But when you have the level of fortune that rivals a small country's gross domestic product, you can customise your boat with whatever ...