The Damsel and the Mermaid

At nineteen, Maylees is way past the age for marriage, so her uncle has chosen a husband for her. She wants to obey but marrying an old man is terrible fate. Whenever she can she escapes to the shore to watch the receding tide reveal the drowned city. Deep in the ocean, Khu does not like any of her mother's choices for a life-partner. She prefers exploring the shallows and the sunken human city far from her people so none ever suspect her forbidden ability to perceive magic.
Maylees never believed in merfolk and their wicked ways, while Khu's people had dire warnings about the evil land-dwellers, so the day they meet changes everything. Despite the warnings, they are drawn to one another until the arrival of Maylees' brother brings disaster with no hope of resolution for either.