Something something... Marine biologist!Robby who works on merman/mermaid research and Jack as the geumpy old merman with a torn fin from a boat accident who has been in Robby's care for like 15+ years.
Jack has a beautiful bay to swim in now that is still safe as he isnt so fast anymore with his injury, and Robby lives in a cottage next to the bay and keeps an eye on Jack while working other projects and he makes qay too many jokes about how they are growing old together.
He is also very aware that Jack sees him as mate, that the shells and rocks and fish he brings Robby is courting behavior and okay maybe they make out on the regular but like... for science, yeah?
Then either Dennis joins as student and Robby is a little jealous Jack seems to like the kid so much and is quite easy around him (aka not as much of a grumpy ol' fish) but Dennis is a good student who clearly cares as much about merman wellbeing as Robby.
Alternatively, Dennis is a young merman found malnourished and tangled in fishing nets and after the main merman population in the rehab facility was pushing him around too much, Robby decides to bring him to Jack's bay.
Jack complaining very loudly about having to share his bay but when robby says he will relocate Dennis then, Jack is quick to argue it would be too stressful and he better stay... he secretly loves having the younger's company and is vedy set on "fattening the kid back up"
And then smthn smthn smthn... turns out Dennis can get pregnant and is carrying Jack's guppy now













