Ascension (Water #1) by Kara Dalkey

Pre-loved paperback 

Nia, a young mermyd of the Bluefin clan, has had one wish all her life -- to be an Avatar in her beloved home of Atlantis. The ten Avatars rule the beautiful and peaceful undersea city alongside the ancient Farworlders, whose magic keeps their world alive. To be an Avatar is an honor and a great responsibility, and Nia dreams of taking her place among the noble ten. Now, at sixteen, Nia has a chance to see her dream come true. Atlantis is choosing its next Avatar, and Nia knows she is supremely qualified. But there is something Nia doesn't know -- if she gets her heart's desire, it could mean the end of her treasured world of Atlantis forever.