Atlantia by Ally Condie
The woman who created the Matched trilogy now brings us Atlantia which is a fantastic book. I was drawn into the world of underwater life and shocked that they weren't mermaids as I originally thought. They're just humans living in a bubble in a basic explanation. However the book has a huge sister betrayal, death and a lot of weird stuff going on that makes it worth reading.
Overall however it's a book that keeps you guessing and second guessing what is going on until the end which almost blew my mind (Gone Girl keeping that number one spot) but I was just kinda let down as well. Trust me it's delivered well, the book is fantastically written. If I had to say more then I think I would give away a hell of a lot of spoilers, therefore I will leave this book in your hands.