The Scorpio Races by Maggie Steifvater
This book has two main characters, Kate Connolly (Puck) and Sean Kendrick. There's one chapter from Puck's view and the the next is Sean's. They both live on a small island where everyone knows everyone else but there's also Capaill Uisce (Water Horses) which are ridden every year in the Scorpio Races. The Capaill Uisce are brutal, bloodthirsty water horses which eat meat. They killed Puck's parents but, to keep herself and her brother alive, she has to take part in the Scorpio Races.
She is the first and only girl to take part so everyone wants her out, especially as she is breaking the unwritten rules again by riding her small island pony instead of a Capaill Uisce. The Scorpio Races are ridden on the beach although lots of the riders are killed because the Capaill Uisce will do anything to get back to their natural habitat - the sea.
Sean Kendrick is also an orphan, as his father was killed by the Capaill Uisce and his mother died, but he is thought to be the only person on the island able to tame the Capaill Uisce. Along with his Capaill Uisce, Corr, he has won the Scorpio Races four times already, but when his and Puck's worlds collide, it all changes.
I love this book because it's an amazingly written story about magical horses and it's not like anything else you'd read about fantasy.