Seven-Tenths is a multifaceted ode to the "sea and its thresholds." Hamilton-Paterson takes the reader on tour with scientists and fishermen, seafaring nomads, and resort developers. He contrasts their exploits with poetic fables and a "pre-GPS" history in which he discusses imaginary islands, sea curses, and sirens. It's meandering and fascinating, and really the most convincing environmentalist text I've read.
Hamilton-Paterson doesn't slap you with heavy-handed preaching; he lets the mystery and importance of the sea shine through the narrative, showing the reader how much humans need the sea (biologically and spiritually) and how close we are to destroying it.