 If you liked
Ao no 6-gou
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 ...then you might like
Suisei no Gargantia
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Some of the themes these two series share would result in leaving spoilers in the recommendation, so I'll stick to pointing out that both series involve an Earth that humans have effectively destroyed and the aftermath of trying to survive that post-apocalyptic world. Suisei no Gargantia takes a softer tone with the themes, but there is a possibility said series is somehow influenced by the older series.
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Recommended by Yemi_Hikari
Both take place on a world submerged underwater with people fighting for supremacy with an alien race. They have many similar ethics and main characters.
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Recommended by QLikesDonuts