The Triton Chronicles: Book One The Guardians

https://www.amazon.com/dp/B016APPGRU

Jake Garrett watched the numbers on his digital watch spinning wildly out of control. Water resistant to 100 meters. This was one of the last things that he remembered as his plane went down over the ocean. Now he has awakened in an underwater city- the home of the Tritons.
To return to the surface, Jake must overcome the treachery of the Siren Queen, a general bent on the destruction of all humans, and a school where they issue nets and spears to the bullies, all in time to stop the attack of a madman who has discovered the secret of the Tritons. 
The Triton Chronicles Book One: The Guardians.