The Above World is deadly and inhabited by a mix of augmented humans (most body forms based on Greek myth), androids of various kinds and a small struggling population of unaltered humans (who we never meet in this book).
Many generations ago, population pressure forced humans to force evolution. Humans invented body forms and tech adaptations to allow them to live in environments inhospitable to our normal bodies. Some took to the skies and live at high altitude. Some took to the deserts (which are apparently more abundant in this future). Two groups went to the sea — one a shark-like people who live in the deep oceans, the Deepfell, and one, Aluna's people, who live in shallow waters and resemble mer-people.
For protection, Aluna's ancestors have hidden their colony. They have stagnated in knowledge and population numbers. Girls are banned from dangerous activities because they are needed for breeding. And they are dying.
They have a breathing implant that allows them to breath either sea-water or air. These implants are failing. But even so, the Elders of Aluna's community will not venture from their protected home to find out why.
Granted, I'm not sure the Elders could venture too far anyway. Mer-people are not born with tails; they take a pill as they become teens that genetically modifies their legs into a tail. So the Elders are sea creatures. Children like Aluna are not.
And so, after causing a ruckus at her tail ceremony and enraging her father (an Elder), she leaves her home and the ocean and goes into the Above World seeking answers. Her best friend, tech genius Hoku, goes with her. Because an adventurer needs a smart side-kick to keep her from killing herself and because he may have important information from their colony's founder in the form of a relic box that has been in his family's possession for generations.
They find out that many of the horror stories about life beyond their cove are true but that friends and allies can be found even among those who seemed to be enemies. They also find out that the true enemy, a madman of epic scale, was once a friend. To defeat him, all the different peoples must overcome their prejudices and work together.
There will be more fun to come. While this book does end (unlike many series contributions) there are more battles to fight before the enemy is defeated. Aluna and her friends will be in the center of it all!