These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Hikari. Is he just a Hormone-Addled Teenager who doesn't know how to express his feelings properly? A Tsundere? An undersea boy who's having a hard time adjusting to living in a new environment, far away from everything he knows, and copes by means of anger? A childhood friend who's just trying to look out for his friend? A misogynist/soon to be domestic abuser who treats his female friend like crap? The fans are VERY divided over this. Although he is much more likeable in later episodes and arguably becomes the series most popular character.
Both a fandom and in-universe version with Akari. She started dating Itaru after his previous wife and her good friend, Miori died. Akari claims that she she felt bad for the Shiodome family and wanted to make them happy by spending time with them and eventually marrying in. Uroko believes that she is just re-phrasing it in a positive light, how it was an "illicit love", she is taking advantage of the Shiodome family's poor emotional state and that she was actually happy when Miori died because then she could finally be with Itaru. Akari thinks that what he says is harsh but she does not deny it. Because of these two different reasons, Akari's popularity ranges from being a Base Breaker to The Scrappy (view Broken Base and Idiot Ball entry for more reasons).
Similarly to Akari's status within the fandom, Itaru ranges from a Base Breaker to The Scrappy. Is Itaru an overall Nice Guy that tries to do the right thing or is he an irresponsible father, boring, incredibly stupid (check out Idiot Ball below) with no personality or his own agency, instead revolving around Akari or Miuna and only serves as a plot device to add forced drama? Due to the wide age gap between him and Akari and the fact that she was still a school student when she fell in love with him, has caused some people to label him a "pedo".
Is Sayu a spunky kid with hilarious lines and good friend to Miuna, or an incredibly annoying Bratty Half-Pint that needs to be properly punished for her stupid actions?
Later episodes have caused all seven leads (as well as the supporting cast) to become this.
Angst? What Angst?: Besides Miuna and Sayu, everyone else seems pretty OK with the oncoming Mysterious Ice Age of Doom. Many of them seem convinced it won't really be that bad, or that it will take such a long time to happen that it doesn't really matter. Sound familiar?
Some people thought Akari's story was well-executed, moved the plot along nicely and really helped with Hikari's character development while others thought it was tedious, a painful chore to watch and often times pointless as well because it did not focus on the main leads, their relationships and worldbuilding as much as it could have. This arc also contributes to Akari's polarising popularity within the fandom.
And in the second half, some fans were not pleased with how the main point of view changes from Hikari to Miuna.
Many people were left feeling unsatisfied with the ending because it left most of the romance open-ended (which is very typical of Okada) and only one couple - Tsumugu/Chisaki became official. Even fans of Tsumugu/Chisaki were annoyed by the lack of romance between them in the final episode and the fact that they didn't get to see a kiss. Others considered the ending too happy and simple because they wanted something bittersweet or another timeskip.
A lot of people seem convinced this show is purposefully trying to be similar to From the New World, which aired the year before. Whether this means, "slightly inspired by" or "full-on ripoff" depends on who you talk to. The two shows are quite distinct from one another but it's hard to really say so early-in. Both series have plenty of worldbuilding, pretty landscapes, a complex Love Dodecahedron, coming-of-age themes and mysteries abound. Come episode 13, both shows now possess the "slaughtering teenage protagonists to create drama" characteristic, though it seems unlikely to take the Kill 'em All route that New World eventually followed.
Narm: How can we take the scene between Akari and Miuna in episode 11 seriously when Miuna's tears look like giant bubbles and aren't streaming down her face like normal tears do?! Chisaki emits similar comically gigantic tears a few episodes later.
Watching the series a second time with the knowledge that Manaka reciprocated Hikari's feelings but didn't want to betray Chisaki offers a more interesting take to her interactions with Hikari and her attempts to "pair" him together with Chisaki (e.g. Episode 5 when she wonders what answers she wants from a sea slug while watching Hikari and Chisaki, and Episodes 9 and 10 after she rejects Hikari when he hugs her).
The Manaka-Hikari-Miuna triangle and Miuna's conflicts in the second half will definitely be seen in a new light when one watches the first half again and takes note of their interactions during that part.
In episode 23, Tsumugu more or less says that he has been in love with Chisaki since the 4thepisode.
Romantic Plot Tumour: Whilst the show is listed as part of the 'romance' genre, it manages to suffer from this, with a really interesting concept becoming clouded by a Love Dodecahedron that makes little logical sense.
After a forced meeting with Uroko and a night of crying alone in her roon, Akari tells Hikari that she will stop seeing her surface boyfriend. She begins to joke with Hikari as usual, only to start softly crying while hugging him.
Although he keeps butting heads with his son Hikari and keeps advising Akari to give up on Miuna's father, their father, Sakishima Tomoru, does really care for them as seen in episode 7. When they both made up their minds to leave Shioshishio and were consequently being forcefully held back by Uroko-sama, Tomori bows and begs for Uroko-sama to show them mercy and let them leave. The episode ends with Hikari and Akari leaving for the surface and Tomori left all alone, having lost his wife, his daughter and his son.