The Voyage to Magical North

Twelve-year-old Brine Seaborne is a girl with a past--if only she could remember what it is. Found alone in a rowboat as a child, clutching a shard of the rare starshell needed for spell-casting, she's spent the past years keeping house for an irritable magician and his obnoxious apprentice, Peter.
When Brine and Peter get themselves into a load of trouble and flee, they blunder into the path of the legendary pirate ship the Onion. Before you can say "pieces of eight," they're up to their ne...