The Siren by Kiera Cass
"You must never do anything that might expose our secret. This means that, in general, you cannot form close bonds with humans. You can speak to us, and you can always commune with the Ocean, but you are deadly to humans. You are, essentially, a weapon. A very beautiful weapon. I won't lie to you, it can be a lonely existence, but once you are done, you get to live. All you have to give, for now, is obedience and time..."
The same speech has been given hundreds of times to hundreds of beautiful girls who enter the sisterhood of sirens. Kahlen has lived by these rules for years now, patiently waiting for the life she can call her own. But when Akinli, a human, enters her world, she can't bring herself to live by the rules anymore. Suddenly the life she's been waiting for doesn't seem nearly as important as the one she's living now.