Graham Hawkes, inventor of the “underwater plane,” made his debut at the Monaco Yacht Show last month in a bid to entice billionaire boat owners to take the plunge.

“This is literally like flying underwater,” said Hawkes, a U.K.-born ocean engineer who has spent decades designing cutting-edge diving suits and submarines. “Once you’ve done that, you don’t want to do anything else.”

Hawkes was one of four submarine vendors attending the Monaco show — one of the world’s top yacht gatherings — for the first time, hawking multimillion-dollar high-tech wizardry they say makes perfect accessories for the wealthy.

U-Boat Worx, Triton Submarines LLC and Seamagine Hydrospace Corp., along with Hawkes Ocean Technologies, are betting the superrich will want to go beyond cruising on luxury boats worth tens of millions of dollars. They see annual sales of private, small luxury submarines going double-digit over the coming decade from a few now.

As the yacht size has stretched — this year saw the launch of a record-holding 590-footer called the Azzam — so has the list of distractions on board. Soaking in a jacuzzi, shooting hoops on a floating court or playing a baby grand Steinway piano no longer cut it. “There is a change in attitude of super-yacht owners,” said Bert Houtman, founder and chairman of the Netherlands-based U-Boat Worx, surveying two of his submarine models on display quai-side in Monaco. “They’re fed up with drinking white wine and riding jet skis so they’re looking for another thrill.”

The submersibles on offer cost from around $1.5 million to $4.2 million depending on their size and underwater range. The current global fleet is estimated at under a couple dozen including on private yachts such as Octopus, owned by Microsoft Corp. co-founder Paul Allen.

There’s one on Necker Island owned by Richard Branson, founder of Virgin Group Ltd., and another on a tourist island off Costa Rica, where a 300-meter dive costs $1,800 per person.

U-Boat’s five models equipped with bubble-shaped acrylic windows can hold between two and five people and sink to between 100 meters and 1,000 meters underwater. Rival Triton, which is based at Vero Beach in Florida, is pushing the depth limit to 1,650 meters for similar battery-powered technology.

Storing an 18,000-pound (8,200-kg) submarine elegantly on a designer yacht can be a challenge. Makers urge owners to have bespoke boats conceived with subs in mind or, alternatively, invest in a “shadow” vessel to transport these types of toys and tenders, smaller speedboats that accompany super-sized yachts.

Private submersibles are “a way of exploring for things that no human has ever seen,” said Marc Deppe, Triton vice president of sales and marketing. “For that you need depth.”

Sharks, hydrothermal vents and sea mounts are among the wonders the more jaded wealthy could admire from an air-conditioned capsule complete with panoramic views and a sound system, according to Deppe.

There are also man-made attractions. U-Boat in July took Russian President Vladimir Putin 60 meters underwater in the Gulf of Finland to see the Oleg, a 19th-century shipwreck.

“A lot of guys who are billionaires have profound financial accomplishments and are now concerned about their legacy,” said Deppe.

Super-yacht professionals are guarded about the identity of their clients so without revealing names, Deppe says this is already happening. The companies themselves train and grant submarine pilot licenses through in-house-designed courses. Safety features include a “dead man” signal that has to be sounded by the pilot every 10 minutes or the submarine automatically returns to the surface.

Hawkes’ underwater flying machines differ from the others. While his resemble soft-edged airplanes with bubble-shaped cockpits, the others are more like UFOs. “They’re like a helicopter, we’re like a Lear jet,” he said. The backdrop is a poster of him and Branson cruising in an underwater plane snapped just before they crossed paths with a great white shark.

“Our goal is to fly and play with the big animals,” he said. “Now that’s magic.”

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