Mermaid on Heels by Karen Daliath

What if Little Mermaid found someone who could breathe underwater like her and she followed him to the forbidden surface to uncover his mystery?

Princess Liriya of Pearltuna had long anticipated her eighteenth birthday. However, when a mysterious mermaid named Faelina—who claims to be her half-sister—crashes her celebration, everything changes. Faelina’s demand for a family artifact leads to a dark enchantment that forces Liriya and her sisters to flee their kingdom.

As they seek refuge in a distant sea, one day, Liriya happens upon a mysterious human who can breathe underwater.
Following him to the forbidden surface to uncover his mystery, she eavesdrops on his conversation with his brother and learns that he is the soon-to-be king of the land, has to pick a bride following his coronation, and that he desires a mermaid bride.
Shocked at her discovery but thrilled about the surface, Liriya brings her sisters up that night. There, they discover their new transformation power and then go about the land, where they learn about an upcoming ball at the palace.

Driven by a strange attraction to meet the prince and the illogical desire to uncover his mystery, the last thing Liriya expects at the ball is he recognizes her as a mermaid and asks her to be one of the elites for his bride. However, with her duty to overthrow Faelina tugging at her heart, can she give him, or even herself, the chance to discover what could blossom between them?