What do you do when already own 5 vacation homes? Buy a custom-built superyacht.

For billionaires, a superyacht is another vacation home. 3PhotoFilipposFilippou/Getty Images

Forget Saint-Jean-Cap-Ferrat in the French Riviera or Kauai island in Hawaii – the best kind of vacation home floats on the sea.

At least, it may be for billionaires who already own other vacation homes on land. Arthur Brouwer, CEO of superyacht builder Heesen Yachts, recently told Shivani Vora of The New York Times that most of Heesen’s clients consider their superyachts their fifth or sixth vacation home.

These clients are typically from the US, Europe, or the Middle East, he said. They either buy custom-built superyachts or pre-build yachts – yachts built “on spec,” Brouwer added. And while some live on their yacht for 30 weeks out of the year, others use their yacht only a few weeks out of the year.

Generally, yachts over 100 feet cost $US1 million per 3.3 feet, according to Forbes. Heesen’s yachts sell for anywhere from $US39 million to over $US112 million, reported Vora, but superyachts can easily exceed $US1 billion.

Read more: I spent 3 years writing about yachts, and owning one takes way more money than you think

But that’s just the beginning of the costs involved in owning a superyacht – owners can expect to spend about 10% of the purchase price annually on operating and maintaining a yacht. That’s $US1 million a year for a $US10 million superyacht, although the exact maintenance costs vary.

Maintenance is just one of the big four costs involved in owning a superyacht, along with fuel, dockage, and yacht crew to run the yacht.

The bigger the boat, the more crew – and salaries – you need. A 130-foot boat with five crew members can cost $US32,500 a month. Deckhands, at the bottom of the totem pole, earn an average of $US3,083 to $US3,574 a month, depending on the boat size. Captains get paid more, earning an average of $US7,750 to $US19,961 a month.

Dockage can cost as much as $US350,000 annually, and fuel as much as $US400,000 annually, including fuel for toys like Jet Skis.