Above World — Jenn Reese

Thirteen-year-old Aluna has lived her entire life under the ocean with the Coral Kampii in the City of Shifting Tides. But after centuries spent hidden from the Above World, her colony's survival is at risk. The Kampii's breathing necklaces are failing, but the elders are unwilling to venture above water to seek answers. Only headstrong Aluna and her friend Hoku are stubborn and bold enough to face the terrors of land to search for way to save their people. But can Aluna's fierce determination and fighting skills and Hoku's tech-savvy keep them safe? Set in a world where overcrowding has led humans to adapt - growing tails to live under the ocean or wings to live on mountains - here is a ride through a future where greed and cruelty have gone unchecked, but the loyalty of friends remains true.