Sea by Sarah Driver 
 
In the sky, the fire spirits dance and ripple. Grandma says they showed our Tribe that I'd be a captain, before I was even born.

Ever since Ma died, Mouse has looked after her little brother, Sparrow, dreaming of her destiny as captain of the Huntress. But now Da's missing, Sparrow is in danger, and a deathly cold is creeping across Trianukka... A sea-churning, beast-chattering, dream-dancing, whale-riding, terrodyl-flying, world-saving adventure.