This is an entry for Finish the Story Contest - Week #56.
Here is 
The air was sticky and they'd been drinking for hours. It was late into the evening of the going away party for Ron and Jenni. They'd been good friends to Sean when he first arrived, but since then they'd grown distant. In fact, it had been awhile since Sean had hung out with anyone. He'd arrived to a room full of people he didn't know, already well on their way to getting drunk.
"It's so great to see you," Jenni said, offering a hug. "Sean, oh man!"
Ron offered a handshake. "Hey man, good to see you one last time."
"Thanks for the invite," Sean replied. "And sorry to see you going."
The room erupted with laughter. A man had slipped and fell on his ass and he was rolling on the ground hollering away. "I'm gonna grab a cigarette," Sean said.
"Oh wait!" Jenni grabbed his arm. "Let me introduce you to everyone!" She proceeded to bring him through the rooms of the townhome they rented for the night's going away party. There was a Jack and a Royal and a Jeremy and a Kim and a Karen and more and Sean soon forgot all those names anyway. He finally got outside for a cigarette, his heart racing and his anxiety peaked.
"Fucking..." he mumbled to himself. "Too many fucking people in that space."
"I know, right?"
The voice startled him. He peaked over and saw a girl sitting with pink dyed hair. "Can I get one of those?"
"You know Jen and Ron?" Sean asked, offering her a cigarette.
"Sure," the girl replied. "We met a few times at church."
"Church?" Sean asked. "I'm incredulous. I didn't know they were Christians."
"Oh, it's not that kind of church." The girl took a drag on her cigarette and the presumed air of mystery she was attempting to paint annoyed him.
"Okay," Sean said. "Well, then, what kind of church is it?"
The door to the porch opened and the music blared from inside. Ron came outside with a group of other party goers, all drunk and cheering.
"I'm telling you," one of the guys was saying. "That girl's totally into me. I'm going to go for it."
"Do it!" Ron said. "We all believe in you man. Believe in yourself!" Then, turning to Sean. "We're going for a swim!" Ron announced. He shouted it again, and the group cheered.
Sean glanced over to the girl, whose name he still didn't know. She was smiling.
They trekked down the dirt road from the pension then through a dirt path. The ground was rocky, volcanic in some spots in that way typical to Jeju. Drunk, most of them stumbled. They passed by a fish farm, the ponds of which were covered in nets. Then a water treatment plant with rumbling generators inside its deep warehouse.
Then they reached a secret, secluded beach. The pink haired girl stripped down, an obsidian necklace dangling from her neck, glowing in the moonlight. She descended into the water.
"It's warm and wonderful!" she said. "Come on."
The group stripped down, and Sean saw many were wearing the same pendant. He began to strip as well, as the idea of warm ocean waters in such a hot night was intoxicating. He went into the water, swimming till it was deep enough to fully submerge.
When he came back up, he heard screaming.
And this is my ending:
The scream had come from Karen. She drew her arm back, for a full swing at some guy's face. They could hear the slap over the waves. "Keep your hands off me you creep!", she yelled.
Sean recognized that drunk man from earlier. Apparently, he had believed in himself way too much. He stood in the water, massaging his cheek, while everyone studiously looked away. At last, he mumbled an apology and staggered back towards the townhome.
But the festive mood was gone.
Jenni approached Ron and whispered something in his ear. He nodded and took her hand. "Sorry about that," he told everyone with a half-smile. "But it's our cue to get an early start on the journey. We're leaving now."
There came quick hugs, and sad farewells, and then, to Sean's surprise, Ron and Jenni started casually walking towards the open sea. Four, five steps, and they went under.
Sean had opened his mouth to speak but thought better of it. It wasn't the realization that everyone present, except for him, was wearing that same obsidian necklace. Rather, it was the lack of gravity. These people looked nothing like a bizarre suicide cult. They were waving half-heartedly, conversing about Kim's frilly bikini and tomorrow's baseball game.
Sean stayed quiet and still and faded into the background.
The sun was about to rise when the pink-haired girl returned to the secret beach. She started removing her clothes. From his hiding place behind the reef, Sean coughed politely before approaching.
"So, about this church of yours."
"What about it", she said, unperturbed.
"I'm guessing, you worship Poseidon? Dagon?"
She rolled her eyes. "Always the Innsmouth joke. We pray to God, man. Just like everyone else."
Sean raised an eyebrow in his best Spock impression.
"But the church does sponsor immigrants," the girl conceded.
Sean lit her a cigarette, which again she accepted. "Is this something you can tell?"
She exhaled a satisfied puff of smoke. "It's not a big deal, kind of an open secret nowadays. There's a vetting process, of course, which can take some years. Less if you got useful skills, like engineering, or biology. Meanwhile, the church teaches the language and customs."
Sean studied the dawn-rimmed waves. "Isn't it cold? Dark?"
She laughed. "It's not the middle ages. There's free wifi." But she was looking fondly at her cigarette. "It depends on how deep you want to reach. Stay in the shallows, you don't need to change much, if at all. Best of both worlds."
She tossed the butt in the sand. "We convene at the harbor every Sunday morning. Newcomers bring their own scuba gear."
Sitting on the reef, Sean closed the page of Amazon's best prices for diving equipment. His phone's background image was a graph of rising surface temperatures. He was getting anxious again. "Too many fucking people, period."
Looking up, he saw a distant whale's tail fin rising from the ocean.
He could swear it waved at him.
