The art again is the best thing about this series, the gorgeous work Igarashi does especially with nature: any ocean creatures, water, weather… he's a little less good with people, interestingly enough, but maybe because the global/cosmic themes are actually more important to him than the characters themselves. Many of the characters are mystical and ethereal and two of them look very much like girls but are boys, to no obvious purpose… but the cast is still interesting, just not as accomplished as the amazing art, which well fits the purposes of the story.
The central idea here blends science (marine biology, in particular) and mysticism to tell a story of something peculiar and concerning about the sea and the disappearance of fish. It also features middle school kids as central characters, so that's an audience issue, to get at the complexity of the known world and communicate that to teens (and older people, too!).
Two kids, Umi and Sora, have a sixth sense, though it is not about ghosts, but about the sea. Instead of seeing dead people or prescience, they can read some levels of the ocean, have a sense of what is going on with it. They also grew up IN it, not just around it, so that's a fantasy element. Ultimately, we learn as we go in the plot that we humans know very little about the sea, we are just scratching the surface and are as arrogant about it as we are about any sort of cultural or ethnic issue. The pace is slow, as Sora goes missing and Umi gets sick and we get some interesting backstory about marine biologist Jim who has known kids like Umi and Sora for forty years. He's not just a hard science guy, in other words, he gets it that the ocean is mosty unknowable and often mysterious. Much of what he is learning as a scientist is informed by Sora's mystical sixth sense, actually. Sora has been working with him. There's a big reveal at the end that is surprising and will make you/me keep reading, of course. This is a really unique manga..