The Water Mage - T. J. Ripley - Bog

Eliot Johnstone is an extraordinarily average sixteen year-old, until the day that a strange package arrives on his doorstep. He is transported in time to a medieval world full of magic and strange creatures, where he befriends an imp named Emmid who teaches him how to be a water mage.
As they battle the evil forces of a powerful death mage intent on capturing him, Eliot uses his knowledge of the modern world to enhance his magic in new and exciting ways. While the mystery of why he has become one of the most powerful water mages in this strange new world eludes him, he nevertheless makes friends and enemies and learns tough lessons about good and evil, life and death, and most importantly, himself.