The Skin of the Sea by Natasha Bowen

Simidele is one of the Mami Wata, mermaids duty-bound to collect the souls of those who die at sea and bless their journeys back home to the Supreme Creator. But when a living boy is thrown overboard a slave ship, Simi saves his life, going against an ancient decree and bringing terrible danger to the mami wata.

Now Simi must journey to the Supreme Creator to make amends - a journey of vengeful gods, treacherous lands and legendary creatures. If she fails, she risks not just the fate of all Mami Wata, but also the world as she knows it.