I read the book "Horizon" by Jenn Reese (Candlewick). It is the third book in the "Above World" series. The book is amazing and incredibly detailed.
It is about Kampii, Equians and Aviars -- humans who are technologically upgraded so they can adapt to life in the sea, desert or sky.
Aluna and Hoku are best friends; they are Kampii who live in the City of the Shifting Tides, in the ocean. They are the main characters, along with their friends Dashiyn (an Equian) and Calli (an Aviar).
Their goal is to defeat their mortal enemy, Karl Strand. He is their enemy because he is an evil human who wants to live forever but uses everyone else's energy to do it.
On the way, they defeat Strand's clones, Scorch and Fathom, with the help of the Serpentine, who are healers, and other warriors.
The book is a combination of science fiction, action and a lot of humor. This is about the best series I have ever read.