I quickly realized that Lewis could probably fund Lake Nona forever simply by selling the art housed on this floating museum. He’s a top collector of modern and contemporary art, and I came nose to nose with a Picasso, a Klimt, a Lucian Freud, a Cézanne, a Chagall, and a Modigliani. I couldn’t help wondering what a freak wave would do to the global art market–and what his insurance bill looks like. The collection will soon have a new home: Lewis is building a larger (300-foot) boat, which will feature a court for his passion, padel tennis (a mix of squash and tennis).

Lewis himself is surprisingly approachable. A slight but fit 78-year-old with a bit of a Cockney accent, he showed me around with a schoolboy’s pride. In addition to the art, the yacht, decorated in royal reds and golds, boasts a starkly modern office, complete with a Bloomberg terminal and multiple trading screens. Lewis trades here every day. He says he tends to bet on liquid currencies such as the pound or the yen. In April he told me that the euro was too high and that the dollar would stay strong for a while. “Being a trader means that you are wrong at the very least three times out of 10,” he says, “and that is very hard.” At this point in his life, Lewis says he doesn’t want to do anything that’s not worth it. Lake Nona, he says, is iconic–and that’s what makes him happiest. “We’re not good at mediocrity,” he says.

This story is from the June 30, 2014 issue of  Fortune.