They go swimming when summer is almost over, the sun deeper on the horizon with every passing day, as much as days can be counted when the sun is in the sky all the time.

The smoothie Minho makes Felix drink tastes like stale fish, but it takes the pain of the shift away, leaving Felix with the foreign feeling of feeling his bones change shape.

Chan is right next to him the entire time. The water is cold, but warmer than Felix remembers, and the bright smiles of the other mermen warm him. Minho, Changbin and Jisung are wearing neoprene suits and full scuba diving outfits. They all swim away from the coast staying on the surface, and when the water is deep enough the humans turn on their back and put on their flippers.

"You ready?" Chan asks Felix, flipping his tail so it catches the light and a ray of water drops fly through the sky.

"Let's go down together," Changbin says. "We'll need to go slower than all of you to adjust to the pressure and all of that. Felix, remember: the important thing is that you breathe the air out of your lungs while we sink."

Felix nods. Swimming and diving comes instinctively to him, and as a merperson he can survive much more pressure than any human, but there's still theory behind it that makes diving into water more pleasant. Felix holds eye contact with Changbin as Changbin puts the mouth piece of his diving regulator between his teeth and starts releasing air from his BCD. Minho and Jisung follow his lead, forming an 'OK' sign with their fingers in the simple language of people who can't speak or hear underwater. They'd explained the different signs to Felix multiple times, but now he can't remember any of them, brain paralysed at the thought of the dark closing in on him.

He reaches out for Chan's hand and lets himself sink.

The familiar noises of the waves crashing on the shore and wind flying over the sea disappear first, then the heartbeat of the sea fills Felix's ears. It's really just his own heart, beating as one with the waves, and for a moment Felix forgets that he's afraid. He barely stops himself from breathing, and then breathing and air is all he can think about. His diaphragm cramps in his need to both breathe and not, and Felix presses his eyes shut and tries to relax.

Unsurprisingly, it doesn't work, but Chan hugs him from behind and he realises the need for air is really just a habit – his heart is beating steady, his gills are filled with water, the water holds him gently, and Chan's tail is curling around his. And this is what does it for him, in the end.

He's not alone.

Felix opens his eyes and sees his family by fate around him. Chan's hands over his stomach, the three humans in a circle with him, Seungmin and Hyunjin below them, looking up at him with big worried eyes, and Jeongin swimming in a big circle around them, light blue scales catching the light like ice.

Felix smiles with his mouth closed, and forms the divers' 'OK' sign with his thumb and forefinger.

They all visibly relax, and Hyunjin grins at him, delighted.

For the next few minutes as they continue to sink slowly to accommodate the humans among them, Felix can only stare. Because his family is beautiful. He's seen his own tail, of course, and Chan's whenever they bathed together, but seeing Jeongin's icy light blue and Seungmin's green and turquoise patterns in the blue of the sea around them? Hyunjin, with his iridescent lilac and purple scales, capturing the rainbow between them?

Felix would like more eyes, thank you.

And it's not dark at all. The light of the sun breaks in the water, but it's only blue, so blue, and the ground is a sandy light brown with rocks and colourful sea grasses. It's bright.

Can I let go of you? Chan asks.

Felix nods and wriggles free of Chan's hold, stretching as long as his body goes. It's nice to have the space. The water temperature is refreshing, but maybe that's just because Felix has been so nervous about this. His insides stop with their somersaults and his muscles gradually relax.

Come on, Lixie! Hyunjin calls. There's some cool sponges growing over there!

Experimentally, Felix starts swimming towards Hyunjin. Hyunjin is impatient and swimming in circles. Felix makes it there in no time. Surprised, he turns around, meaning to check up on Chan. He twists with too much force, unintentionally throwing himself into a spin. Spreading his arms he tries to float, which is exactly when the water takes him with it. For a moment, he doesn't know where up and where down is, but then Hyunjin grabs his hand and steadies him.

You got this, Hyunjin says, though Felix definitely has not got this. You're putting too much force into your movements. You don't need to. Just look at Binnie and Sungie.

Felix turns toward them. They look weirdly out of place with their scuba gear and goggles. They're moving slowly, bodies horizontal with their arms in front of them, like they're just lying around. They move by using their feet and the flippers they wear. It makes sense, though; they shouldn't exert themselves as their bodies cope with the change in pressure and their supply of oxygen is limited.

Chan is lazily swimming next to them, eyes fixed on Felix.

So, I'll try to move minimally? Felix asks. The words feel weird leaving his mouth, and he doesn't notice he's opened his mouth and therefore his lungs for the water. To his surprise, it doesn't hurt. It's a foreign, uncomfortable feeling, and there's salt on his tongue now, but it's – not bad.

Yeah, Hyunjin grins at him. Unless you want to swim very fast or jump up and out of the water, just relax. Feel the current.

Felix nods. He has no idea what Hyunjin means with that, but he can relax. He copies the way Hyunjin moves, keeping his arms by his sides and under him.

And it's easy after that. Felix has been swimming all his life, after all. It's the space that makes him unsure, the possibility of swimming in one line for as long as he wants that gives him anxiety. But he's not alone. The sponges Hyunjin shows him are very average underwater growths but Hyunjin pretends they're some kind of treasure because they're the most interesting thing around, and it's adorable.

Jisung even brought a waterproof camera, and to Felix's endless embarrassment he takes pictures of them as Hyunjin points at random things and Felix tries to move elegantly.

It's not long before Jeongin gets bored and requests Felix learns how to do more than float around, but this only makes Felix reveal his superpower: a perfect jump out of the water and a perfect salto in the air and a perfect dive back into the waves.

And instead of the slight humiliation Felix always felt when he did this in front of an audience at the aquarium, he only feels pride and satisfaction at the dumbfounded and impressed expressions greeting him. (And well, Chan is always looking at him with love.)

Then Hyunjin and Seungmin get very close to his face and tell him he needs to teach them immediately, and Felix can see Minho and Changbin laughing so much they swallow water. It's fine, they're experienced divers and know how to deal with it. Felix just hopes Jisung got that on camera.

Even Chan wants to learn how to do the flip, and after a while their jumping attracts some of the seals from the next bay. They keep their distance, curious at the merpeople with nothing better to do than exercise, but it's not like they're doing anything except lying in the sun.

Then the humans need to go back on land, and they gather in a loose circle as they slowly rise, focused on the dive computers on their wrists to keep an eye on the pressure around them.

When they all have their heads above the water, Jisung whoops and throws his arms around Felix with a bright grin. He's clumsy, held back by his gear, and it pokes Felix in his ribs. "You did it! You were amazing!"

Felix, occupied with coughing the water out of his lungs, grimaces and returns the smile. "I'd like to do this again sometime."

This earns him more whoops and yells of encouragement, and as they swim back to the coast where they stacked towels and warm clothes, Chan grabs Felix's hand.

Chan's smile is the sweetest yet, quiet and warm and adoring, and Felix squeezes his hand.

Chan holds him as he shifts back into his human skin once they reach the shore, painless as the shift is with Minho's smoothie – it still involves his skin changing and bones fusing.

Chan kisses him when it's over and exhaustion settles in Felix's bones, and Felix kisses him back to the sound of the others around them cooing and calling them cute.

It feels like home.