Hong Kong (CNN) — Stepping onto The World feels like boarding a 21st-century Titanic, such is its scale and grandeur.

Sitting majestically in Hong Kong's Victoria Harbor, this gleaming 644 feet-long white vessel is the largest, and probably most exclusive, "residential yacht" on earth.

Home to the world's only full-sized at-sea tennis court, a 7,000-square foot spa and fitness center, and 12,000-bottle wine collection, this ship has visited 1,213 ports and sailed 641,000 nautical miles.

This year alone it has undertaken expeditions to two of the most remote regions on earth -- the Ross Sea, in Antarctica, and Melanesia, near New Guinea.

But don't get too excited.

None of the suites on this 12-deck beauty are available to rent.

Dubbed a "condo cruise liner," every one of the 165 luxury apartments on board -- worth between $3 million for a studio and $15 million for a three-bedroom pad -- are owned by residents who must have a net worth of $10 million. At least.

To get on board, you'll need an invite.

Since launching in 2002, The World has sailed 641,000 nautical miles. The World

A rough sailing

When The World was launched 15 years ago, it nearly floundered.

"Initially, the ship was partially owned by a hotel company," Lillian Veri, a Canadian who has owned a residence on The World for nearly 10 years, tells CNN.

The sixth floor of the ship, she explains, had initially been reserved for hotel rooms, the rental income from which was intended to subsidize the residential side of the business.

"Well, it just didn't work out that way," Veri says.

All interior designs for apartments on board the ship must be approved by a committee before being executed. Sam Rohn/The World

In 2003, the business model changed. There would be no tourists on board. Just residents of a very high net worth.

Related content New US embassies make an architectural statement

It became -- and remains -- the only entirely residential cruise liner in the world.

All residents are shareholders who vote on everything from the ship's route each year, to the type of fuel used and the Christmas decorations.

"The people who buy here are successful in one way or another. Lawyers, doctors, architects, entrepreneurs," says Veri. "They have opinions on how things should be run."

The change in business plan worked.

By 2006, all of the residences had sold out.

No place for Oprah

During CNN's tour of the boat's facilities, various residents float by.

Looking like passengers of a regular cruise ship, they all cheerfully greet by name our guide Lisa Spiller, who joined as residential director of The World six months ago.

Everyone who passes, I realize, is at least a multi-millionaire. Just how rich, I ask Spiller -- who herself is dripping in what appear to be diamonds -- are residents of The World?

Lisa Spiller, residential director of The World “The type of people who buy here have private jets. They collect art. This is not their only residence.”

She smiles. "Let's just say the type of people who buy here have private jets. They collect art. This is not their only residence."

The wrong kind of success, however, could see a wealthy candidate vetoed by the vetting committee.

"I don't think that Oprah Winfrey would be allowed to buy here," explains Veri, as we chat in her three-bedroom apartment. It boasts a wonderful wrap-around terrace that today has an unobstructed view of the Hong Kong Island skyline.

"There's a code of confidentiality and privacy ... We don't want paparazzi here. This (boat) is a refuge, a sanctuary.

"You will never find out who else lives here."

Inside one of the residences on The World. The World

Today, 142 unidentified families reside on the ship, who all have undergone a strict vetting process before being allowed to buy. Roughly half of those on board are North American, about 45 are European and another 20 are South African. The average age is 64 years old.

General manager Sandra Mooney says that, on average, most residents spend about six months a year on board the ship, which flies the Bahamas flag and adheres to that country's rules when in international waters. Occupancy peaks at Christmas, when many guests invite their families and friends on board.

Still, a ship that was built for 600 people, says Mooney -- hotel rooms have a higher occupancy per square foot than residences -- only ever has 330 maximum on board.

Travel without leaving home

That The World is impressive there can be no doubt.

On a clear night far out at sea, residents can choose to sleep under the stars on a collection of "Bali beds." Each apartment receives complimentary turn-down twice a day. Bvlgari toiletries appear in the bathrooms as if by magic.

There is wifi coverage wherever the boat is, doctors on board, and even a pilates teacher on hand.

But wouldn't individuals with such fabulous wealth, and who seem to value privacy so highly, prefer to buy and travel on their own yacht?

The appeal, explains Veri, lies in the adventurous itineraries The World's staff put together.

"I don't have the creativity these people have. It's a lot of work to put all that stuff together."

By the end of 2017, for example, the ship will have visited Vanuatu, the Solomon Islands, Hawaii, Shanghai, Hong Kong, Canada, Alaska, Mexico and Central America, rounding off in Miami.

Three times per year The World undertakes "expeditions" -- voyages into unusual destinations, which are joined by leading environmentalists and academics, who through a series of lectures, forum breakfasts and field trips stimulate educational discussions about the given destination.

The World in the Ross Sea. The World

"We did, a long time ago, (a trip to) Madagascar," says Veri. "We had a National Geographic photographer, an anthropologist, a marine biologist. It's like being at a university for a week. That you can't get anywhere else."

Plus, there's the sense of community, says Mooney, recalling numerous parties the residents have thrown on board for one another.

"We feel this is our family, too," says Mooney with a sparkling smile, of the relationship between the staff and crew. "We have our family at home and we have our family on board. It's lovely."

The World is not enough

The World, it seems, is not enough. Other cruise ship operators are now eying a slice of the luxury floating city market.

Launching in 2021, The Utopia is a five-star residential cruise liner set to rival The World.

The Utopia ship Utopia residences

It will have cost an estimated $980 million to build, according to its owners, and its 190 residences will be listed for between $4 million to $36 million. The ship's route will be tailored to tie in with key events on the global calendar, such as Cannes Film Festival, Monaco Grand Prix, the Olympics, fashion weeks, the Melbourne Cup, and Rio de Janeiro's Carnival.

"Utopia will be a place for annual meetings of first ladies, philanthropists, Nobel laureates, festivals of thinkers, and missions where world leaders are bringing together conservatives and liberals to solve pressing issues and brokering peace treaties among feuding ethnic groups and cultures," The Utopia's PR team tell CNN over email.

Unlike The World, however, there will also be 165 hotel rooms on board.

An artist's impression of the finished The Marquette ship. River Cruises

Launching in 2019, The Marquette is a residential-only river cruise ship, with apartments on board selling for the more modest price range of between $310,000 and $1.9 million. The project was founded by David Nelson, who has lived on a houseboat on the Mississippi river in the United States for 29 years.

For Mooney, however, no ship will rival The World.

Related content Innovative designs for communal living

"People think of (The World) as a cruise ship," she says. "It's not."

The World, she explains, is a unique floating city of like-minded individuals with a passion for travel and learning, which over the course of 15 years has become home to a bonded family. That sort of chemistry is hard to replicate.