AN: Here's more of this universe! Thank you for all the follows, favorites, and reviews for this story!

Disclaimer: I don't own Castle.

Kate and Rick stand side by side, staring at the ocean, toes digging in the sand.

"Ready for a swim?" she asks, grinning.

"Yep," he returns the smile as they wade into the water until they're both under. She forms his oxygen bubble after giving him a quick kiss.

"Hold on tight," she reminds him as her arms wrap around him and they're off, the speed at which the couple goes turning everything into a blur.

Soon, the water clears, telling Rick that they've reached their destination. They swim along the usual pathway until they reach the pristine buildings of Atlantis. The ancient city doesn't change much in structure, but the merpeople come and go often.

"I need to see Mathias for a minute," Kate says as the duo swims along, the merpeople eyeing them. While Rick's been there numerous times, he's still considered an oddity in the community. The people are nice enough, but highly suspicious of him.

Kate nods hello to the two guards, both of them tossing friendly smiles her way and giving Rick fist bumps. When they swim into the throne room, Mathias sits in his seat, eyes on the duo as they enter.

"Hello Katherine, Richard." He gives them both nods. "I hope everything is well."

"Yes," Kate says. She then proceeds to give him an update on the surface world, which isn't much.

"I'm glad to see that everything has settled down up there." Eyes turning toward Rick, Mathias raises his eyebrows. "And how have you been Richard?"

Rick smiles easily, not bothered by the implication behind the question. "Great! About to have another book launch, in fact."

"Ah, yes," Mathias says with amusement. "Katherine told me you had finally finished your newest book. She's absolutely raved about what she's read."

"Have not," Kate mumbles, but her cheeks turn pink.

"Well, that's excellent to hear." The author smirks. "Perhaps I can give you a copy somehow."

The king waves a hand. "Don't bother. I have no interest in the human world, particularly the highly depraved part."

Rick sighs, knowing there's no way to change the elderly man's mind. "Of course."

"Anyway, I have several appointments for today. Thank you for coming, Katherine. I will see you soon."

"See you next time, your Highness," she says, both of them bowing before leaving the throne room.

"He still doesn't like me," Rick grumbles.

"He doesn't trust any human, don't take it personally." She grabs his hand as they glide through the city. "Besides, as long as he doesn't interfere with us, who cares."

"Right," Rick mutters. He's still bummed that the king can't warm up to him, regardless of what the author does.

"Anyway, let's visit the grotto."

"Okay!" Rick grins, happy to see that she's feeling sentimental today. It works perfectly for him.

They swim, more like float, along the area, enjoying the scenery of colorful plants and fish, holding hands. He knows it's a unique version of a romantic walk, and he wouldn't have it any other way.

And he hopes to do it for the rest of his life.

They arrive at the grotto and he takes his usual seat, glancing at the pictures. He knows the stories behind each of them now, Kate having weaved each tale on various visits. His imagination still runs wild sometimes, though, and he imagines different backgrounds for some of the images.

"I still miss my mom sometimes when I'm here, usually when I'm alone," she says, sitting next to him. "But you always make it better."

He would kiss her, but his need for oxygen prevents him doing so.

"And I hope I continue to make you happy." He smiles. "After all," he starts, digging into the pocket of his swim trunks. "We are soulmates." Holding out his hand, he opens the velvet box that he's been holding onto for months. He hears her gasp slightly as he does a semi-kneel.

"Kate, I love you, and want to be with you for the rest of our lives, well mine at least." They both chuckle. "I know we're soulmates, but I want this to be your choice. So, will you do me the honor of marrying me and becoming my partner for life?"

She grins, her eyes shining with happiness. "Yes," she murmurs. "Yes!"

With that, he slides the ring on her finger and unable to resist, kisses her fiercely and twirls her around. Kate laughs, one of the most beautiful sounds he's ever heard. However, he can feel his airways tightening and so does Kate as she makes quick work to construct another bubble.

"Come on, let's get going before your airways really burst," she says, leading him out of the grotto.

They speed swim back to surface, right where they started. As soon as they break the water, they kiss for real, long and slow.

"Come on, let's dry off and head back," Kate says, resting her forehead against his.

"I'm sure my mother already has champagne ready and halfway finished."

His partner rolls her eyes. "Har har. Give your mother a bit of a break."

"Yes, dear."

She elbows him as he carries her to a nearby rock. They're far away from prying eyes, so she's safe. As they wait for her legs to return, he notices her gazing at her ring, playing with it in the sun. It glitters as much as her tail does. Huh. He didn't think she'd be that focused on it.

"Everything okay?" he asks.

The smile she gives him is genuine. "Yeah, just I'd never thought I'd find my soulmate. Now, I can't believe I'm wearing an engagement ring."

He cups her face with his hands, looking into her eyes, hoping she sees the love in his. "I'll try to make sure you believe it every day." He places his lips on hers, hands beginning to roam. She chuckles.

"No beach sex. Sand is no fun to wash off," she reminds him. Right. The last time they made love on the beach they were finding sand in the most unfortunate places days after.

"Later then," he murmurs before gathering their belongings. "Come on, let's go home."

"Yeah," she states, wrapping an arm around his waist. "Let's go home." He glances at the ring on her finger, and then her. With her hair dripping wet, sticking to her face, her smiling at him looking utterly blissful, it's one of the most beautiful sights he's ever seen.

He knows he's already home.