Destria Waves by Lily Raine
After transforming a bear into a human boy Ana struggles with the idea that she is not human. When Ana discovers a secret lair hidden beneath the waves where the demons are regrouping, Ana realizes she has a terrible choice to make. Sacrifice her life and the life of the mermaids who fuel the underwater power source, or escape and allow the demons free reign to destroy the human world. But if Ana chooses to accept her immortality, then the life of her cousins and boyfriend, Daniel, will forever be compromised.