Jimin is a curious merman who finds a chess board at the bottom of the sea and after weeks of trying to figure out its purpose decides to go and find a human who can explain it to him.

Jeongguk is a rich kid on vacation/in the rebellious phase and cruises around on his boat to get away from responsibilities, annoying best friends and just people in general. He also very much does not believe in mermaids and stuff like that because he is way too old for fairytales.

Taehyung is just angry at Jeongguk leaving him behind without a word but it all changes when he meets Jimin who has a damn fine tail and probably can help him out because his pet goldfish Hector may or may not have depressions.

Yoongi is one of those incredibly rare mer dryads almost nobody knows about and he prays it stays that way because he really just wants to chill in his own nice pond but people keep interrupting him and he almost accidentally kills Jimin more than once.

Hoseok is Taehyung's goldfish except that he really is something like a fairy merman who plays pranks on Taehyung in his tiny seahorse-sized form but his so-called owner still didn't realize he is very much not a goldfish, does not even look remotely close to one so maybe that's the reason of him sulking.

Namjoon is busy with all of the academic degrees he is trying to juggle so he never visits Jeongguk to meet the 'fricken beautiful fishman-hottie' himself and slowly but surely gets worried about Jeongguk's mental health.

And inbetween all that mess of awe, astounishement and chaos Seokjin is doing his best to take care of his bunch of idiots who forget to eat more than once while fawning over Jimin's tail - all the while hoping to not go insane in the process.

or

the Jikook merman au nobody asked for