There is this girl, who lived all of her life in a submarine where her and other teens, the crew, don't age. They also can't go out, because if the sea water touches you, you become a huge fish and with time you forget who you are.
So she investigates, I think, with a friend of hers. They end up in a boat, then a big ship. She finds out if she jumps from a high tree she can grow feathers and can fly (I think?).
There are many things I don't remember, but in the end she had became a fish with her friend, then they found an island and turned back human again, but with blue skin.