Whispers of the Deep by Emma Hamm

One of the many engineers that keeps Beta, the city under the sea, running, Mira only wants to make her family proud and to prove herself worthy. She knows the mysterious city better than anyone and it’s her dream to help it flourish.
But then, on a solo job in an abandoned section of the city, she encounters a monster of legend. An undine. A dangerous merman from an ancient civilization, long forgotten.

Arges has fought his entire life for his people. With deadly creatures under his control, he plans to eradicate Beta once and for all to protect his kind and their peaceful way of life. But when a human woman saves him, she unknowingly creates a bond between them, one he can’t ignore. Even though her flaming red hair haunts his dreams, he needs her for information on the undine’s enemies.
So he steals her. Keeps her. Feeds her. Only to realize their bond is far deeper than captor and captive. He cannot let her go—but he cannot keep her under the sea. In a battle to determine if love can survive a war beneath the waves, it will be their decision that changes the tides.