In the beginning I thought it was this, but I wasn't sure until the end - but by shell divers they're talking about 海女 or 'sea women'. Pearl divers! Especially when he mentioned at the end about diving with only a loincloth, which is true. Way back when, the women would dive topless, but eventually they adopted this white uniform thing for appearances. And they dive with that sort of barrel looking thing, which you see a few times (especially when they show the granny)
Fun fact: the song of the sea, the singing, and whistling (overall, the whole song motif) throughout this story also plays to another traditional thing that these shell divers used to do (and still do, because it's actually kind of important to do). Because they would dive very, very deep, and they would dive so often in one period, they developed a technique of releasing air from their lungs once they resurfaced. They release it in a slow, unique sounding whistle. This is done specifically to ease any decompression sickness they might get, if not totally avoid it.
So that whistling probably ties directly into that in the story. Also, women are traditionally pearl divers. That trend probably started in the past, when women were thought to be able to hold their breath underwater longer, and their extra subcutaneous fat would allow them to brave the colder temperatures better than men.
Anyways, I really loved the story. Cute and beautiful, and the way it ties into an old, Japanese tradition was very nice. The interplay between that and fantasy like mermaids was great. Also, I was so sad when Shizuku went away and everyone forgot her! But Minato had too much determination. I thought it was so cute that she spent probably over a year going to different beaches and going for a swim, looking for her. At the end when she was able to confess her feelings was such a cute way to end things, and I had a whole bunch of questions and thoughts I was going to type up, but I think Fujisawa answered them all at the end - they said they intended to have 'Shizuku stay on land with Minato until she died' and that Shizuku would stay young and beautiful, even as Minato aged, because she is a mermaid. Very interesting. Very cute! Very bittersweet, for Shizuku, but it is better to have loved and lost, as they say. I'm really glad they got a happy ending.
I had more thoughts, especially about the sister, the granny, and some stuff that might have happened in the past (that might have influenced the sisters attitudes towards humans and mermaids, love, and so on) but I already wrote a huge novel so I'll stop now. As always, thank you for the upload!
The Ama pearl divers are also a big study for scientists, as over a few generations and conditioning, their skull bones are just a bit different in order to deal with the pressure. Curiously, breathing tests indicate that they are actually worse than normal humans when it comes to breathing out, the normal way we measure breath control.