The Scorpio Races by Maggie Stiefvater

Nineteen-year-old returning champion Sean Kendrick competes against Puck Connolly, the first girl ever to ride in the annual Scorpio Races, both trying to keep hold of their dangerous water horses long enough to make it to the finish line.  c2011
Subjects:
Fantasy Fiction.
Racing. -- Fiction.
Orphans -- Fiction.
Horses -- Fiction.
Love -- Fiction.