Sea Bound

(This is set in a post-apocalyptic world where the majority of the world is now covered in water...oh and there are mermaids...and monster and what not)
Serenity is one of the last humans on earth (and we're not even sure she is human), she has been raised by mermaids all her life in, what is now an uneasy truce but was once something far darker, now the sea witch, Esmeralda, has escaped again (she does this a lot) and Serenity has been charged with the duty of finding the witch. Throw in Serenity finding more humans, one of which is a very hot guy who likes Serenity a bit more than he should considering he only just met her, and an equally hot mermaid prince (who hates being referred to as a mermaid) who also like Serenity. Oh and there's some extra stuff but that's spoilers.