Tide Stealers (The Sunken Kingdom #2)
Asa and Rollo are royal children. Their parents, King Sigurd and the Star Queen, are dead. Flood, the evil court magician, has taken over and used his powers to drown the kingdom. Asa and Rollo are the only hop remaining for the people of the Star Lands.
In Tide Stealers, underwater outlaws steal Asa's magical Moonstone Star and throw it away. she and Rollo must find it, but the sea giants have it now!