You change your initial response, and accept the mer's request, agreeing to a complicated partnership. They're overjoyed! With every visit, they get more and more affectionate, with even more gifts for you than usual. Instead of staying in the water's edge, they'd drag themselves up on land to be beside you, and sometimes you'd go in the water with them, swimming together and playing in the waves.
As the seasons change and get colder, they sign to tell you they would be leaving. They explain that in the winter, their particular subspecies would swim into the deeper parts of the water, to remain in the water trapped heat and warm each other up. Both of you sad at this fact, you have a heartfelt temporary goodbye, embracing one another as you promise to wait.
And you do. You wait. And wait. You wait for months. Those close to you get very concerned, questioning why you have become so sad. You aren't entirely sure. You knew you cared greatly for them, but you didn't realise just how much they meant to you.
Finally, spring came by once again, and you're greated to the cove by the face of your very excited partner, so so glad to see you again. This day is one of the best of your life, as you cuddle in the sand, dance in the waves, float side by side, and warm up in a fluffy dryrobe when the water got too cold and your fingers started pruning. You sit together, watching the sun set on the ocean before you, glittering on the surface, like it was waving the day goodbye.
You have to prepare to say goodbye again. You promise you'd be back tomorrow, but they aren't ready to accept that. They had missed you far too much. You struggle to get them to let go, but you don't want them to either. They get so tense, their hair becomes electric with sadness, and you get stung as though from a jellyfish with one tiny touch.
They apologise profusely, rubbing their fist over their heart so many times, they practically rub their skin raw. You keep insisting it was fine, that you should just go home and put it in some warm water, but you're suddenly caught off guard when they take your hand in their mouth, softly massaging the sting with their tongue.
You know it isn't meant to, but... it feels nice. Very nice. Their warm, wet mouth, sucking at your hand, careful not to scratch you with their teeth... That feels amazing. You'd already forgotten your sting.
You coax them off your hand, insisting you're fine. But you have a new idea. An idea that would mean you can stay longer. Making sure you're hidden, you completely undress, confusing your partner, who wasn't aware you could actually do that. Then you slide back into the water, humming under your breath at the cold touch on your heated body. You take their webbed hand, and lead them out of the way of the public beach, swimming to a small clearing beyond the rocks.
By the time you reach the area, they're already hot and bothered, flustered and panting, as much as they try to hide it. You don't know why at first, until it hits you - you were getting more and more excited about your plans the closer you got. They were breathing those fluids in, and that was turning them on uncontrollably.
You encourage them to stop trying to hide their slowly unsheathing member. That you want this. That was all they needed to hear before they embrace you yet again, this time, sliding their long, salty tongue in your mouth. They weren't sure you'd like that at first, but after you moan into their mouth, they completely lost that train of thought, sliding it down deeper into your throat, loving your taste, wishing you'd moan more.
You hold them close to you, feeling their growing member softly poke into your stomach. You slide a hand between you and them, and gently take it in your palm, making them growl and moan in an unmistakably inhuman way. You massage circles into their soft yet slimey cock, determined to make it come out all the way it possibly can, while their claws start to dig into your back uncontrollably, desperate for more. Their hips buck slightly, and you tease them, telling them it's okay if they need to hump like a land animal. After all, you'd practically domesticated them as such.
They resist at first, but soon find themself unable to, barely digging the points of their teeth into your neck as they mindlessly slide their cock in and out of your hand, drooling all over your shoulder and back. You aren't even moving your arm anymore, since they're so distracted doing that themselves. Eventually though, you snap them out of their steady trance, pulling them halfway onto land before swapping your hand with your mouth. This custom isn't really normal for their species, so at first they get very confused as to what you're doing.
Once you take it in your mouth though? Oh ho, they get it alright. They understand now. They growl and howl in pleasure, unable to lay still, swishing their tail and thrusting up their hips as best they can. Though you realise quickly that they're a bit uncomfortable at the same time, laying on their back with their dorsal fins, so you flip them onto their stomach instead, with them now on top of you and thrusting down into your throat. This is clearly MUCH better, as they get even louder and more desperate, gently but firmly grasping at your hair and trying so desperately to get it even deeper, until eventually, they hit their limit, unexpectedly exploding down the back of your throat, filling your mouth with their salty cum.
You take a moment to catch your breath and give them a chance to regain their strength, until you're both ready to go again. This time, they want to give you that feeling, so you welcome them to do what they want with you. They take your unique anatomy into consideration, realising you don't have a sheath or anything like that, before noticing your hole. They give it a curious lick, recognising the taste from earlier, and noticing your whine of pleasure. Aha.
They put their tongue to work, licking over and around your hole, barely pressing in no matter how much you moan and push back into them. They take their sweet time before finally honouring your wishes, sliding their hot tongue an inch deeper with every stroke, going agonisingly slow. In a small teasing pullout, they realise how much you cry in pleasure at one particular spot their tongue had pressed. They experiment, trying that spot again, and that gets even more of a reaction. Bingo.
They go in an overwhelming circle of a routine, pushing in, sliding over your spot on the way out. Pushing in, sliding over on the way out. They don't even take the chance to breathe, as their gills are in the water enough to get oxygen that way, so it's a never ending chain of deep, deep pleasure, and heavenly pressure to your most sensitive spot.
It gets too overwhelming eventually, and you can feel the heat of orgasm coming up fast. Without a second thought, you suddenly grab them by their tendrils, pressing their face into your genitals as you cum all over them, crying out in deep, deep bliss. They clearly find this equally arousing, as that small action is enough to break them again, and they sing out in unison to you as they cum all over themselves and the foamy sand.
You two had stayed up all night, but that's okay. You snuggle up nice and warm on the sand, calm and tired in the aftermath, embracing as you watch the sun rise in a similar way to having watched it set what seemed like days before. You sit in silence, but you break it to ask if they'd mind just spending next winter in your house with the heater on. They happily agree, never wanting to spend so long apart again.
Part 1 is in the comments.