With a price tag of $627 million and a length of 590 feet, Azzam is bigger than a U.S. Navy destroyer.

Not much is known about the world's longest and most expensive yacht. It was launched last year from a shipyard in Germany, and is thought to belong to United Arab Emirates President Khalifa bin Zayed bin Sultan Al Nahyan -- who is worth nearly $18 billion, according to Wealth-X.

In addition to the hefty upfront costs, the annual operating costs on yachts is roughly 10% of the purchase price, said Pedro Muller, strategy director at The Superyacht Group.

Game: What super rich celebrity do you spend like?

For Azzam, that's nearly $63 million a year on crew, fuel, docking, repairs and other costs.