Veiled By Desire by Candace Robinson

Sometimes fate has other plans…

In Laith, when the moons are high, Tavarra gains the ability to walk the land, losing her seahorse-like tail. But should she remain out of the water, a curse will consume her, turning her into a beast with sharp fangs and long claws. A creature that, on some nights, becomes a rampaging monster.
Rhona and her village are under a sinister leader’s control. He took Rhona from the man she loves, stripped away her water ability, then forces her to retrieve a coveted prism that will increase his powers.
When Tavarra and Rhona cross paths, they agree to help each other succeed. They then embark on separate missions in hopes of breaking Tavarra’s curse and saving Rhona’s loved ones. But with an untamable beast inside Tavarra, and Rhona having to wander through a dangerous forest, nothing is certain…