Book of Oceans (Diadem Series)

Born into nobility, Helaine can't get over the fact that Jenna, a mere peasant on their home world, is her equal. Their constant bickering convinces the four magic-users to take a much-needed vacation. Hoping for a relaxing time at the beach, they go to Brine, a water-filled world with floating islands and telepathic sea creatures.

What's supposed to be a relaxing holiday turns into chaos when pirates attack the helpless natives and capture Jenna and Pixel. Using magic is difficult on this outer-rim planet and Pixel's fate becomes grim when he is thrown overboard to a bloodthirsty sea monster. Meanwhile, Jenna-inexperienced in battle and separated from her friends-must rely on innovation and instinct to outwit the pirates, who are forcing her and the other captives to tunnel dangerously close to the sweltering core of an active volcano.