The Mermaid That's Sweetest by C.Swallow
Mermaid Lily killed her best friend, by accident, when she was eight years old. Ever since, she's been too haunted to touch the ocean again. She's not only plagued by her past, but a reoccurring dream, which was once peaceful and is now turning... strange.
She doesn't know it yet, but she's connected to a darkness that could kill her, and that darkness is her mate; an Erebos. They are the kind of Mermaid that feasts on the blood of their own kind - and they are infamous for being the most dangerous threat to the island where she lives.