«These Wicked Waters» by Emily Layne

A centuries-old curse plagues the island of Viaii Nisi and an ancient enemy lurks beneath the depths of the surrounding water.
Annie Mayfield has heard all the stories and rumors about the island that is now home to the brand new Mayfield Villa resort, and she is definitely not psyched about having to spend her summer working there. The island’s name alone—Viaii Nisi, or violent island—is enough to make any sane person seriously reconsider it as a vacation destination. Then there are the mysterious deaths of every previous owner! It’s a history Annie’s mother is quick to shrug off, but when a guest goes missing on opening night, Annie really starts to get the creeps.
And then Annie makes a truly terrifying discovery: ruins filled with bones and one skeleton that seems to be half human and half fish. Intrigued by the strange remains and determined to help find the missing guest, Annie channels her inner Nancy Drew—minus the skirt and pearls, of course—in an attempt to uncover the truth about Viaii Nisi. But that truth is beyond anything she could ever have imagined. With her mother in complete denial and local officials unconcerned, Annie finds she’ll have to face her biggest fears if she’s to attempt to save everyone she loves.