Pirates book one: Spies

Sarah Williams thinks her father rules everything but finds out Pirates rule the sea, but one night a boy her age washes up on shore with ripped and ragged clothes. He doesn't remember how he got there and what had happened before. But soon she becomes suspicious of the boy because he seems to know everything to answer the questions her dad throws at him. But could he really be a spies?
(Ps can be a little violent for younger kids)