Soul of the Deep by Natasha Bowen

One life. One choice. One sacrifice.

To save those closest to her, Simi sacrificed everything - her freedom, her life as mami wata, and the boy she loves. Now she serves a new god, watching over the Land of the Dead at the bottom of the ocean.

But the tide of change is coming and when Simi discovers demons have entered the human realm bent on destruction, she must break her vow and return to the world above the water. The fate of the world lies in her hands.