Lost Heir by G. Bailey

Not all Gods belong to the sea…
Everleigh of Calais thought her heart belonged to the throne she was given, but the gods have other ideas.
After the war of Calais, the last thing her people wanted was for their queen to be killed and sent to the deep sea of souls. But in the sea, Everly might find her home in the shape of a handsome, sarcastic, slightly insane god with stunning eyes, and a kind soul.
In the year she has been missing, a new queen has risen in the sea and wants the crown that is not hers and back on land, her best friend is struggling to hold the seven islands together.
The sea wants her heart,
The Mermaid King believes her heart is his,
And her protector will fight the very waves of the sea to bring Everly back.
Two crowns. Two worlds. But only one queen.