The Battle for Atlantis — Mark Terry

When 12-year-old Peter Namaka nearly drowns while riding his boogie-board off Maui, he is rescued by a girl who can breathe underwater. She takes Peter to her undersea city of Kam'Loa, where Peter is tested to see if he is a Summoner -- a child of one of the Polynesian gods, able to summon at least one species of sea animal. To everyone's amazement, Peter isn't just a Summoner, he is The Summoner, the Heir of Ar'Tur. A famous prophecy suggests that the Heir of Ar'tur might be able to restore Atlantis, which is now divided into 49 separate and warring states. Teaming up with Gwen, merman Lance, and his buddy Bobby, Peter will take his magical sword and encounter gods, demigods and monsters as he discover his own powers and faces Prince Mordred in a battle for Kam'Loa.