Return to Isle of the Shallows by Debra A. White

Drew Barret is about to start a new school after being suspended from his last one for fighting. He hopes he can survive the semester without running into bullies or becoming one himself, but his fiery temper keeps getting him into trouble. When he's snatched away from the lake near his home by a mythical monster, he finds himself trapped in a fantastical world, held captive by a maniacal beast, putting his true nature to the test. With his family and best friend, Trev, venturing into the world of Aqualor to find him, he relies on their support to help the Arvaks, Winged-Groshin and Lamera find peace and freedom in their homeland. Together, they experience a wild adventure and witness Drew's journey of self-discovery as he battles to control the rage within him.