These incredible vessels are owned and enjoyed by celebrities, royalty and titans of industry.
The smooth running of these colossal boats depends on a troop of dedicated workers. And among the most diligent are stewardesses.
They call themselves "glorified housekeepers" but they're much more than that.
They're also entertainers, cleaners, cocktail mixologists, waiters and even yoga teachers.
Chief Stewardess, Gemma Hulbert
, has been working as a "stew" for almost seven years.
"We're caretakers for the guests -- anything and everything they need is done before they need it," the 25-year-old explains, adding that guest requests at times can be quite unusual.
Once, she says, a guest only wanted to bathe in half a tub of hot water that was then topped off with half a bottle of none-other than Fiji water and half a cup of baby oil.
"That was so much fun to clean from the marble after," Hulbert quips.
"I've also had a charter guest in the past who purchased a vintage Hermes bag in the States so our purser (the officer on board who keeps the accounts) had to organize a private jet to go to the States from Monaco to pick up the bag and bring it back so she could take it to a gala the next day," she adds.
also works as a superyacht stewardess and on-board yoga teacher -- something she calls both exhausting and rewarding.
"They (guests) are used to luxuries -- they're desensitized to having help 24/7 and having people take care of them 24/7," the 33-year-old tells CNN from Monaco during one of her rare days off.
"Of course, they have expectations as I do if I go to a restaurant, but as far as people being the clients I have worked with or worked for have truly only been grateful and gracious.
"You can always tell who has come down from generations of wealth -- they just carry themselves with a bit of grace and class versus maybe someone who has just come into money for the first time and they have a little bit of their nose in the air, little bit of an attitude about them.
"No judgment of course! It's just interesting."
'Celebrities aren't the norm'
Nielsen has worked on both private and chartered yachts -- including the award-winning 312ft motor yacht, Kismet
, owned by billionaire Shahid Khan, owner of the Jacksonville Jaguars and Fulham FC. It reportedly charted guests Beyonce and Jay-Z
"I get asked about what celebrities I've met and I've been lucky to have met a few but that's not the norm from what I have found -- at least from my experience. Of course, there are celebrities who own yachts but it's not as prevalent as really wealthy entrepreneurs who have been smart with their investments and can afford to throw money into a big hull in the water," Nielsen says.
"It may be someone who has a specific brand or product that has exploded or someone who makes a specialized product for cars. One of my previous bosses brought the concept of loyalty rewards to his country so all the grocery stores in his country now have loyal reward programs."
It's common practice for stewardess' to sign non-disclosure agreements to protect guest identities. Something Nielsen says is out of privacy and respect.
Reconnecting guests via yoga
Nielsen compares her hours to that of doctors.
"Basically we're on call 24/7," Nielsen explains. Generally, she works a 13 hour day -- with a two-hour break during the day, and a nine-hour break overnight. She does everything from maintaining the boat, serving breakfast, lunch and dinner and entertaining guests by offering them yoga classes.
"The yachting industry is very materialistic and very luxurious and a lot of beauty is to be said about it but you can lose track of your heart and soul and the things that really matter in life and I feel honored to be able to reconnect people to that part of them."