The Beacon and the Brine by Katherine McIntyre

A desperate human dreamer. A sea monster bound by contract. A forbidden romance that could set them free…or end in destruction.
After one mistake too many, Elrich’s family is marrying him off.

Arielle Triton is his future bride-to-be, gorgeous, eligible, and from a wealthy family with connections to New Atlantis…who also hides the fact they’re merfolk. Except there’s no spark toward Arielle, none of the soul-searing romance Elrich dreamed of.
Yet, Ursuline, the serious cecaelia who’s their family lawyer? From the moment he meets them, he’s drowning in their sharp gaze, living for their low chuckles, their dry wit. But Ursuline is bound to the Triton family, just as Elrich is bound to marry someone he doesn’t love. And as those feelings grow stronger for Ursuline, the tug-of-war between duty and desire become unbearable.
As the truth behind the Triton family wealth surfaces, Elrich can no longer ignore the weighted whispers or disappearances from the staff. However, learning too many secrets in the Triton household is deadly, and if he and Ursuline stand a chance of survival, he’s clutching onto one hope.
Some contracts are destined to be broken.