Adrift in Currents Clean and Clear by Seanan McGuire

Giant turtles, impossible ships, and tidal rivers ridden by a Drowned girl in search of a family in the latest in the bestselling Hugo and Nebula Award-Winning Wayward Children series from Seanan McGuire.
Nadya had three mothers: the one who bore her, the country that poisoned her, and the one who adopted her.
Nadya never considered herself less than whole, not until her adoptive parents fitted her with a prosthetic arm against her will, seeking to replace the one she’d been missing from birth.

It was cumbersome; it was uncomfortable; it was wrong.
It wasn’t her.
Frustrated and unable to express why, Nadya began to wander, until the day she fell through a door into Belyrreka, the Land Beneath the Lake–and found herself in a world of water, filled with child-eating amphibians, majestic giant turtles, and impossible ships that sailed as happily beneath the surface as on top. In Belyyreka, she found herself understood for who she was: a Drowned Girl, who had made her way to her real home, accepted by the river and its people.
But even in Belyyreka, there are dangers, and trials, and Nadya would soon find herself fighting to keep hold of everything she had come to treasure.