The Scorpio Races by Maggie Steifvater

Steifvater, M. (2011). The scorpio races. New York: Scholastic Press.
It happens at the start of every November: the Scorpio Races. Riders attempt to keep hold of their water horses long enough to make it to the finish line. Some riders live. Others die. At age nineteen, Sean Kendrick is the returning champion. He is a young man of few words, and if he has any fears, he keeps them buried deep, where no one else can see them. Puck Connolly is different. She never meant to ride in the Scorpio Races. But fate hasn’t given her much of a chance. So she enters the competition — the first girl ever to do so. She is in no way prepared for what is going to happen. Puck lives on a small island with her two brothers. Their parents were killed by the water horses or capaill uisce. Sean befriends her and when she enters the race with her regular horse, Sean and his horse lose to protect Puck and her horse.  She wins and saves her house. The series, The Wolves of Mercy Falls, about a girl who falls in love with a werewolf is a great read as well.