I was rambling on about an AU like this on Tumblr last night and just had to make it happen.
"Kurt! Where are you going?" King Burt called from his study.
"I just wanted to take a quick swim around the city for a little while, Father. I can't sleep," Kurt said, hoping his father couldn't see him wince in the dim light of the hallway. He'd been hoping to sneak out undetected this late at night.
"Just be careful, alright? There have been reports of sharks in the area, and I believe the humans' ships are due to return soon," Burt said, returning to his work. "If they see you-"
"-Yes, yes, I know, Father," Kurt said, trying to hide his impatience. "I'll be safe, I'm only going into the coral forest a little ways." He swam off before Burt could say anything else – or worse, tell him to take a guard.
Kurt swam in the direction of the coral forest for a moment or two before shifting course and heading for the open sea, where lights above indicated that there were humans present. He stayed low, looking for the red-painted bottom of the particular ship he wanted and smiling ecstatically when he saw it moored off to the side of the crowd by a convenient rock outcropping. Kurt pushed himself onto the low rock in the center, perfectly hidden by the bigger one in front of it but able to see through a chink in between some others.
"Hey, hey!" a voice – Kurt's favorite voice – shouted from the ship. "Watch it! I don't want to go overboard in the dark!"
"If you'd just tell me who you fancy, I wouldn't have to chase you around the ship like a damned rat!" another voice yelled back. "C'mon, Blaine, don't be a holdout!"
"I told you, David, I don't fancy anyone at the moment," Blaine said. "We've been at sea for months, who am I supposed to fancy? Wes?"
A voice that must be the mysterious Wes cried out in indignation while the others around Blaine laughed.
"Fair enough," David replied. "But you know your parents are going to start their talk of betrothals again once we arrive back at the palace."
"I'm aware," Blaine said, sighing, as the ambient noise around him shuffled off, probably in search of more ale. "And I understand that they want to see that our line is carried on after we lost Coop in that storm, but I – I don't know. I haven't found anyone I wish to marry yet, I suppose. Am I being foolish?"
"You do have to rule with them for the rest of your lives," David said. "It's not foolish to want to spend your time with someone you can at least like, if not love."
"Loving the person I marry would be ideal," Blaine said. "I think I'm still hoping that fairy tales are real and when I see the one, I'll simply know and everything will work out perfectly, even though I'm about as likely to find true love as I am to grow gills and a tail."
"Well, you do have that odd wrinkle on your neck," David teased, making Blaine reach for his nape in a panic and Kurt giggle quietly from his perch. "I think I'm going to go refill my tankard before Trent drinks all the ale himself. Shall I bring you any?"
"No, thank you, David," Blaine said, watching his friend walk off. He climbed onto the large, flat side of the ship and stretched out, staring at the stars. "Whoever you are, future partner of mine, I hope you like looking at the night sky as much as I do."
Kurt flipped onto his back, too, searching for the big, blue-tinted star that his father had told him was Poseidon's mark. "I doubt I'm the one you're thinking of, Blaine, but I like stargazing, too," he whispered. "Nighttime is the only time my people can surface without the fear of being seen." He glanced down at his deep violet colored tail, wondering what it would be like to have legs.
Before Kurt could go on, Blaine's voice drifted out over the water again, this time singing a slow, unfamiliar tune. He sang of hope and sadness and love, and Kurt was captivated. In fact, Kurt was so entranced that he almost didn't notice that the sun was rising.
"Oh no," he said, noting the pink tones to the sky. "Father's going to be so angry with me." He quickly flipped into the water and swam for home, promising himself he'd come back one last time to hear Blaine's voice before his ship returned to shore. Kurt might get in trouble for it, but there was simply no way he could miss his last chance to hear that smooth, incredible voice.
Sneaking out the next night was, unsurprisingly, more difficult than usual.
Kurt had been grounded the second he arrived back at the castle, and the tides were predicting a massive storm was going to roll in, which made everyone more alert. Fortunately, Kurt always had a back-up plan.
There was a secret passageway in his chambers that led to a grotto just outside of the castle walls. Kurt had played in it often as a child, making it one of the first places anyone checked whenever he was missing – at least, until he'd mostly outgrown it three years ago, when he was fifteen.
"Please, Poseidon, let me fit through these tunnels, if only for tonight," Kurt muttered as he shifted his bookcase away from the entrance. He wasn't entirely sure Poseidon was real, but he thought it couldn't hurt him to ask for a blessing just this once, especially since he felt a strange sense of urgency about getting to Blaine that night. He swam carefully through the passageway, taking care to make himself as small as possible when he reached the narrower tunnels. There were a few close fits, but he managed to make it out of the castle without alerting his father or any of the guards.
Once he was free, Kurt sped through the rapidly-darkening waters to the outcropping where Blaine's ship was anchored, hoping against hope he would still be there, at least for a few last minutes. Just as Kurt surfaced behind the ship, a lightning bolt struck, hitting the – mast? Kurt wasn't sure what the different parts of ships were called – and splitting the wood with a loud crack.
"No!" Kurt gasped as people aboard the Warbler started screaming. He could see men running for the wheel and the sails, trying to steer it as best they could, when suddenly one man lost his grip on the sail he controlled just as Blaine was approaching. The force of the blow knocked Blaine overboard.
Kurt instantly dived below, scanning the nearly black waters for a glimpse of Blaine. He had almost given up hope when a snatch of white caught his gaze, much deeper down than he was and off to his left a ways.
"Blaine?" Kurt called out, swimming as fast as he could for the flash of white he'd seen. As he got closer, he could see that yes, Blaine was wearing the white cloth, but he wasn't moving. In fact, he was starting to turn a similar color. Kurt wrapped his arms around Blaine's middle, forcing a stream of bubbles out of his mouth. "I've got you. I'll get you back to the shore."
Kurt pushed himself even harder than before, uncertain of where they would turn up once they surfaced but knowing that Blaine needed air if he was going to survive.
Kurt wouldn't accept any other outcomes.
The swim felt like it took longer than any other journey of Kurt's life, but soon he and Blaine broke free of the water, somewhat near a long stretch of shoreline. Kurt could see a huge building in the distance, noting that it looked vaguely like his own castle, but smoother, and made out of an unfamiliar material. He used the last of his strength to get Blaine on the sand, collapsing a few feet away from him once Blaine was securely out of the water.
"Please, please breathe, Blaine," he said, barely audible over the crashing surf. "We've made it this far, please don't give up on me now."
There were a few moments of tense silence where Blaine's chest remained entirely, devastatingly still. Kurt almost gave into the tears he could feel forming, but then Blaine gave a small, precious wheeze. The wheeze turned into a cough that shot water out of his mouth, which turned into a raspy inhale, which turned into a series of gasps. Kurt knew he should get back in the ocean before Blaine could notice he was there, but he was too focused on making sure Blaine would be all right to move.
"Oh my God," Blaine said once he was able to breathe again. "It's you."
"It's – it's me?" Kurt parroted, confused.
"I thought I'd seen a boy with fins swimming away from my ship the other day, but I figured I was going crazy with loneliness," Blaine explained. "I'm Blaine, by the way. Prince Blaine, of Dalton."
"My name is Kurt," Kurt said. "Prince Kurt, of the ocean. Please don't – don't..."
"Don't what?" Blaine asked, looking intently at Kurt.
"Don't...capture me and take me away?" Kurt said lamely. "My father's always warned me never to reveal myself to humans, especially after my mother never came home, but now I've really gone and mucked it up."
"You saved my life, Prince Kurt," Blaine said. "I would not repay you by taking away your freedom."
"Just Kurt is fine," Kurt said. "And thank you, Prince Blaine, for being so kind. I heard you singing the other night and assumed you were a good man, but it's nice to know for certain."
"You can call me Blaine," Blaine said, pushing himself closer to Kurt. "This may sound strange, but I feel as though...as though we were meant to find each other. As though you were supposed to hear my song and return."
"As though I was supposed to be there to save you?" Kurt continued Blaine's line of thought. "The whole way to your ship tonight, I felt like I needed to go faster, like there was a reason I had to be there tonight."
"And there was," Blaine said, cupping Kurt's cheek. His hands felt warm and rough against Kurt's tender skin. "We had to meet."
"Yes," Kurt said, no louder than a breath. "Yes, we did."
Blaine took that moment to lean in and kiss Kurt softly, sweetly, no harder than a piece of kelp might brush his tail. Kurt wrapped his arms around Blaine's neck and pushed the kiss a little deeper, feeling a tingling sensation throughout his body. Golden light blazed, shining through even his closed eyelids, and when he opened them and broke the kiss, he noticed-
"Legs!" Kurt said, looking down and wiggling the two long limbs that were now in the place of his tail. "We kissed and now I have legs!"
"And to think I believed fairy tales weren't real," Blaine said, full of awe. He stood up shakily before offering a hand down to Kurt and pulling him upright as well, letting Kurt cling to his shoulders as he got used to the feeling of standing. Just then, a wave rolled into shore, climbing all the way up to where Kurt and Blaine and-
"Ow!" Blaine cried out as they both dropped downwards. "Why did – oh my God!"
"Now we have tails?" Kurt asked, more confused than ever. "Yours is very pretty, by the way."
"Thank you," Blaine said politely, and really, his shimmery navy blue tail was quite nice. "I suppose that kiss must have worked both ways."
"So when we're in the water, we're mermen, but when we're on land, we're humans?" Kurt said, hoping he'd made sense of what was happening to them.
"I think so, yes," Blaine said.
"Well, that does make it easier to tell my father what's happened," Kurt said. "I was worried about what he would do if I never came home."
"And I was wondering how to tell my parents about your heritage," Blaine said. "I am allowed to tell them, aren't I?"
"I think that would be acceptable," Kurt said. "After all, I'm your true love, aren't I?"
"I'd like to think so, but that smirk on your face makes me believe you know something I don't," Blaine said, pushing himself out of the water and watching as his legs returned.
Kurt followed suit before saying, "I happened to overhear you talking with David the other night, when you said that you were about as likely to find true love as you were to grow gills and a tail. I think you just accomplished both, darling."
"And to think I thought I was being flippant," Blaine said, smiling at the memory. "Well, my love, whose parents do you think we should tell first?"
"My father is likely tearing the sea apart in search of me by now."
"And my parents aren't expecting me for another day yet," Blaine said. "Very well, let's go talk to your father first. I've always wanted to see life underwater."
"Just a second, my dear," Kurt said, taking a couple of shaky steps toward Blaine. He looped his arms around Blaine's neck and gave him another kiss. "In case we run into any sharks before we make it back."
"But that won't happen, will it? Kurt...Kurt!"
Kurt simply flapped his fins in the sunlight, waiting for his adorable, gullible Blaine to dive in after him.