Between New Year’s Day and New Year’s Eve of 2015, Jerry Shine dove up and down the East Coast of North America, through winter storms and summer heat waves, in crystal-clear water and muddy muck, night and day, always from the beach. The book “A Year Underwater” is his record of that year, of creatures mating, spawning, hiding and making meals of one another, of encounters with squadrons of squid, 20-pound lobsters, sharks, critters inhabiting tiny worlds almost too small to see, and a diver or two that you probably already know.