337216 Deepwater King - The Deepwater Trilogy by Claire McKenna

Since losing her great love to the Queen of the Sainted Isles, Arden must fulfil an impossible promise before she can return home - she must complete the dangerous Rite that will return Jonah's spirit to the abyssal Court of the Deepwater King. This sets her off on a journey far out at sea to find believers of the old religion on the oil-slick and mysterious islands beyond the horizon. But such a responsibility will not come without sacrifice, for the Deepwater folk who worship the King require the most desperate payments the soul, and with one man Arden may have to pay the greatest price of all… Astonishingly original, with world-building to rival the depths of the ocean, McKenna has drawn a rich tale of longing and courage - penning the perfect oceanic steampunk fantasy.