OK look at the cover: Helen Dummore:Ingo
I would never have picked up this book - it's obviously a girly book about mermaids isn't it?
No, absolutely not. It's not in the slightest bit girly, though would appeal to boys and girls.
Gripping story, powerfully written, complex characters and plots. But as far as teenage themes (sex, drugs, bullying, violence, etc.) there's nothing inappropriate for fluent readers as young as 8 or 9.
Oh, whilst I'm here can I slip in another one? I thoroughly enjoyed Ally Kennen's The Beast recently too. It's not suitable for younger readers of a sensitive disposition. (Ds1 chose not to read it late night in bed.) And definitely wins my vote for the most attention-grabbing start for a teenage novel recently: