Secrets Among the Tides by Aleera Anaya Ceres

The only life Maisie Fauna knows is one of poverty and tragedy. Years of war killed off her friends and neighbors one by one. So when the crown’s soldiers arrive to take their pick of able-bodied mer, it’s no surprise that the Captain of the Royal Guard takes an interest in her.
Only, it’s not so she can fight.
The Princess of Thalassar is missing and to save the kingdom, they’ll need a lookalike. They’ll need Maisie. And to help the poor, forgotten mer of Thalassar, she’ll do whatever it takes.
She didn’t take into account that she’d be suffering the scrutiny of her guard, the attractions of a crown prince, the temptations of the kingdom’s most wanted criminal, or the various attempts on her life.
The kingdom’s secrets are deadlier than she could have ever imagined and Maisie soon discovers that the reason the princess disappeared was because she knew too much…
…and someone wanted her dead.