Random detail I noticed while working on this—the map on page two appears to show that Tasomira is about a thousand kilometers away from the coast of Nii-jima, which would mean a few things:
1. Jo swam a thousand kilometers laterally (not to mention the vertical distance) before transforming into a human.
2. Mermaids having a border guard at that distance would make the perimeter near impossible to reliably patrol, even if they only patrolled the southern coast of Japan.
3. There is absolutely no way they would have ever realistically found Jo and Yuki.
Yes, yes, a bit odd that I can suspend my disbelief enough to accept that there are mermaids and they have a magical pill that'll turn you into a human (and vice-versa), but I can't believe that the leading characters would be found by a mermaid border patrol off the coast of Japan. Oh well.