Ascension (Water #1)

Nia, a young mermyd of the Bluefin clan, has had one wish all her life -- to be Avatar in her beloved home of Atlantis. To be one of the ten Avatars that rule the undersea city is an honour and a great responsibility. Now at 16, Nia has a chance to see her dream come true. Kara Dalkey's Water is a fantasy trilogy that combines the legends of the Knights of the Round Table and the lost city of Atlantis.