The Princess and the Pearl — E.D. Baker

When Princess Annie's father and uncle fall ill and no doctors in Treecrest can cure them, there's only one person who might be able to help -- a healer who lives in Skull Cove, all the way on the other side of the world. To journey there, Annie and Liam must set sail on dangerous seas full of sea monsters and magical creatures. There, Annie finds out that the only cure for her father is a giant pearl that's been stolen and held captive by a protective sea witch. Can Annie find the pearl before it's too late?