Guardians of the Gates — Laura D. Bastian

Thirteen-year-old merman Exander wants to leave Atlantis to mingle with the humans and explore the world above. Talia, his best friend, wishes she was a guardian of a gate leading to the surface. She'd keep those nasty humans out of her beloved city of Atlantis. When Talia accidentally short circuits a gate, Exander is trapped on the wrong side, getting a crash course in navigating the human world. As Talia fights to help Exander come home, she learns someone is closing the other gates, and it's destroying the magic of Atlantis. If they can't convince the magical creatures of the world to save the gates, the destruction of Atlantis will only be the beginning.