The Last Olympian by Rick Riordan

Last Olympian is a fantasy-adventure novel based on Greek mythology by Rick Riordan, published on May 5, 2009. The Last Olympian is the fifth and final novel Percy Jackson & the Olympians series and serves as the direct sequel to The Battle of the Labyrinth. The Last Olympian revolves around the demigod Perseus Jackson as he leads his friends in a last stand to protect Mount Olympus.
In order to try to head off Kronos' approach by sea, Percy Jackson and Beckendorf go to blow up his ship, the Princess Andromeda. However, Kronos is not caught off guard, and Beckendorf sacrifices his life to destroy the ship, while Percy Jackson dives overboard and is knocked out by Kronos' scythe when battling him on the ship. Percy Jackson is awakened by Tyson (a cyclops and Percy's half-brother). He finds that he is in his father Poseidon's underwater palace, which is under siege by the forces of the Titan Oceanus. Percy wants to stay and help fight with his father, but Poseidon sends Percy back to Camp Half-Blood. At Camp Half Blood, they learn about the mission and Silena, Beckendorf's girlfriend, is in tears. At a meeting Percy hears the Great Prophecy for the first time. Then Percy Jackson mentions that they have a spy, someone who has been informing the Titans for years, but they put it aside for now. The Ares cabin had a fight with the Apollo cabin, and is so furious that they wish not to fight in the war... (Summary by Wikipedia)