Nagi no Asukara is a cute little story about people living under the see and how our main protagonists, Manaka, Hikari, Chisaki and Kaname are forced to go to school on the surface. From there on, it touches on the relationship between people from the sea and people from the surface and their differences and similarities.
I had a hard time writing a summary for this because in the end, a lot happened but it was so subtle that it doesn't feel like that at all, quite the contrary.
I don't think the plot of Nagi no Asukara was very interesting. It was mysterious sure, that world had a lot of possibilities too but somehow, everything felt normal. It's a pretty strange feeling, especially considering that everything that happened was supernatural in some way or another (except for the love stories). It is especially visible in one of the very first scenes of the anime. It opens on our group of protagonists waiting for everyone to show up and leave for school, except that to leave for school they have to swim up to the surface. Yet, while they mention the school on the surface, it's only to complain about having to go there and keeping their old uniforms to protest. Somehow this very first scene gives the tone of the anime and sets it up as a real slice of life, despite all the supernatural happenings.
I guess we should take that as a sign of proper and well-done writing. If an anime can make us feel like people living underwater and gods are a natural thing, you can't argue. Nagi no Asukara was well written.
In the end though, I think that the most important thing in this anime was the relationships. Not only between the different people but also between the different individualities. It was interesting to watch Nagi no Asukara explore the differences between two people that everything opposes. I liked to see everyone slowly (very slowly) warming up to each other. It feels very close to real life and seeing everyone get along at the end was definitely heartwarming.
It's the kind of story that's usually supported by its characters. Yet they were extremely generic. No one really stood out. I would have a hard time telling you who was my favorite. Actually I cannot tell you who was my favorite. It might have been Tsumugu but I am not sure. Either way, everyone was pretty generic (except maybe Uroko-sama but he is, after all, the incarnation of a god's scale) but at the same time, the fact that everyone was pretty generic made everyone pretty believable.