Series: Nope, stand alone
When the air Above grew too polluted, Earth's inhabitants retreated to Below. Rio Conway lives in the underwater city of Atlantia with her sister, Bay. If she went Above, she wouldn't have to hide her siren powers, but how could she ever leave Bay? After their mother's murder, they're all they've got. The Conwy sisters vow to stay together. Then, on choosing day, Bay abandons Rio without a word of explanation.
Shocked and alone, Rio combs Atlantia for answers to her sister's leaving and her Mother's death. Her only lead is Maire, the siren aunt her Mother warned her never to trust. And that was before Mother died on Maire's doorstep. Though Maire claims she has a way to reunite her nieces, Rio's sure the only to join Bay is to break Atlantia's greatest law and swim above.
Rio's strongest relationships are with her mother (dead) and Bay (absent for most of the story). Her beautiful siren's voice lets her control any normal human, but to conceal them, she's forced to speak in a flat, dull voice that prevents her from making friends. After a whole lifetime of this she has no desire for social interaction outside of her family. There's a love interest, a boy named True, but their romance is extremely understated due to Rio's personality. Some readers will applaud this as a "sisters before misters" story. Others will find the characters flat as Rio's voice.
The world building is fantastic. Atlantia is a fully fleshed out setting that incorporates ocean folklore and myths in a way that makes the fantastical familiar and believable. The city also has it's own religion, which enriches the story.
Atlantia has an assertive but introverted heroine. The writing's beautiful in a watery sort of way: smooth and flowing, but sometimes it all looks the same. The setting's definitely worth reading, but the characters will make and break the story for different readers.