As the cruise around Brooklyn’s Sheepshead Bay wound to a close, and passengers downed the last of their free champagne, the captain appeared on deck with news: The tide was too low, and the boat could not dock for at least another hour.

“I’ll let you know when we can dock,” he told the crowd. “Until then, keep loving each other, and keep f***ing each other!”

The passengers cheered. That was, in fact, exactly what they had come to do. They were members of Chemistry – New York’s private, bi-monthly sex party, which was being held on this particular evening aboard a private yacht.

As the captain finished his announcement, several passengers scampered upstairs to pick up where they had left off. Others simply lingered by the bar, snagging one final drink.

The captain settled himself in a seat on the stern deck, staring out across the water, his arms draped around two scantily-clad brunettes.

A Chemistry party is – to borrow a cliché – not your parents’ swingers party. The crowd at Chemistry skews younger, towards the mid-20s demographic. While the party caters primarily to couples, few of them are married.

This is by design: Co-founder “KennyBlunt” said he and his partner “SheilaMonster” founded the party in 2006 as a way to get away from the “gauche” sex parties overrunning New York nightlife at the time.

To keep Chemistry parties out of typical swingers territory, attendees are warned before entry that this is not just an excuse to hook up. Instead, Chemistry is billed as “a sensual gala that encourages the organic development and respectful connections between spirits”.

The yacht set sail from Sheepshead Bay, near Coney Island (Getty Images)

The atmosphere at the party certainly suggested something a little more spiritual: The cabin of the boat was dimly lit, and strung with tea lights and strips of breezy white fabric. Attendees came dressed to the nines in sequins, sparkles, and clingy dresses. That night, a few donned sailor costumes to fit the theme.

While the parties cater to the Brooklyn crowd, the couples I spoke to came from as far away as Mexico and Denmark. Their personalities ran the gamut from buttoned-up lawyers to man-bun sporting artists. They also varied by ability: One woman attended in a wheelchair, pushed around by her partner.

I located one couple I'd met at a meet-and-greet a few days before the party – an event the organisers cheekily refer to as "Foreplay". When I first saw the couple at the meet-and-greet, they were both wearing thick-rimmed glasses; she, a cardigan buttoned all the way to the top. By the end of the boat party, she was wearing nothing but underwear.

Another couple I spoke with – two young divorcees from New Jersey – told me this was the best sex party they’d ever seen. To be fair, they hadn’t actually seen many sex parties before – But they had gone to the "orgy dome" at Burning Man, and assured me this was much, much better.

The couples chatted nervously as the party warmed up, listening to the live band play smooth jazz on the bow, or watching costumed burlesque performers wind their way through the crowd.

The truly tantalising part of Chemistry didn't come until much later in the night, after hours of idle chatter and some loosening up beside the bar. As the night wore on, couples began to make eyes at each other from across the deck, and disappear slowly into the play spaces. Titanic-style, the windows of the boat began to steam up.

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The pacing, according to co-founder Kenny, is intentional.

“We need to make a party that we would enjoy going to; that was sexy, but had the build that a good, romantic sexual evening had,” he told me. “It wasn’t something that would make people feel uncomfortable. We really wanted to make something that flowed naturally.”

The slower pace also makes people feel a little safer when taking off their clothes in front of strangers. Donna Ferrato – a veteran nightlife photographer was shooting that evening's soiree for New York Magazine – told me she had never seen a sex party where women were more comfortable, or more in control.

Consent is a key part of the Chemistry experience. Attendees are instructed before arrival that they must obtain a clear, verbal “yes” to engage in sexual activity with anyone. They are also instructed to clearly say, “no,” if they don’t feel comfortable. ("If you're a maybe... still say ‘no’,” the invite advises.) In the women’s bathroom, next to an oversize bottle of mouthwash, was a sign reading: “Articulate your boundaries clearly”.

The sexual “play spaces,” as they are called – actually just open rooms strewn with mattresses and lights – are set up to accommodate varying levels of comfort. All the play spaces I saw were located on the upper deck, away from the main party – though with a fairly clear view from the upper deck, where curious couples could go to sip their drinks and cast glances inside.

For more reserved couples, a windowless room in the back was completely walled off, to provide whatever semblance of privacy there can be among strangers getting naked together.

But even in the best-designed spaces, sleeping with other people is complicated. Pretty much every couple at Chemistry recommended talking to your partner – extensively – before attending; laying out ground rules, comfort levels, and even hopes for the night.

When done carefully, the experience can be liberating. The Burning Man couple was nearly bouncing off the walls that night, both of them grinning ear to ear. Neither of their previous partners, they said, would ever have wanted to do something like this.

Another couple, a professional pair in their late twenties, said going to Chemistry parties had opened up their relationship like nothing else. They had been dating for three years before coming to their first party, and never discussed things like moving in together, getting married, or having children. But after talking out their sexual preferences, they suddenly felt more comfortable discussing other things as well. Now, they were apartment hunting.

Another couple was just at the start of their relationship – in fact, they were hesitant to call it a relationship at all. But they were excited to start their journey together in such an intense, emotionally demanding way. The man described locking eyes with his partner at the same time she was having sex with someone else, and still feeling an intense connection to her. That, he said, signified a truly strong bond.

The Independent spoke to a number of couples who said the experience brought them closer together (Getty Images/iStockphoto)

Plus, the whole thing was a new and exciting thing for them to do together.

“It’s like going skydiving” he said.

But, just like skydiving, the thrill of having sex with other people is also exhausting. By the time the captain made his announcement, most people had already made their way downstairs, carrying bundles of now-shed clothing in their hands.

As the boat made its way to the dock, party-goers lingered on the bow, flopping onto the scattered couches and lighting up cigarettes. One woman started talking loudly about her brunch plans for the next day.