Pearl in the Ice

Marina's father, a naval captain, has been away for most of her life - certainly since her mother died - and yet Marina feels the pull of an ocean she's never known. When sent to boarding school to learn to be a lady, Marina decides instead to stow away on her father's ship. A perilous voyage awaits - but where are they sailing to and why, and what has it to do with the dark shape in the deep that seems to be following them?