Poseidia (The Poseidia Series #1)

In the darkest depths of the ocean, the ancient city of Poseidia has secretly thrived, protected by a sentient dome. Its inhabitants, genetically engineered shape-shifters and protectors of the ocean, teeter on the brink of extinction as the reach of humanity grows.
When a pregnant Anna Ryan is murdered at sea, her life doesn’t end—it begins. She awakens in this unknown world in a new body, enhanced with iridescent skin, fins, and the ability to breath underwater.
Grieving the loss of her baby and her humanity, Anna vows to make her murderer pay, but it's no easy task to brave the wide ocean alone. Anna finds an ally in Roman, a giant, scarred mystery of a man, and together they forge a bond sharing former human pasts. The easy part is convincing Roman to help her retrieve a cherished locket—an item she believes essential to endure the challenges of a new life—but she carelessly endangers Poseidia.
Can Anna set aside everything she’s known and embrace all she’s ever wanted? What is the cost of happiness?