At first, they were elusive, popping heads or fins just above the water line, then disappearing from sight and diving deep, as if playing a little game of hide-and-seek. Rolling through the busy, working harbor on the double-decker Seagull II, a small sightseeing boat run by the local historical society, the salty stretch was busy with activity, barges pushing by, a dredge making a channel along the sandy bottom, ferries shuttling cars back and forth to the Bolivar Peninsula. “We have as many as 1,200 dolphins around the island, and our driver here, he’s a dolphin whisperer,” said the friendly guide, Brian, as we reached the end of the harbor. With bigger waters ahead, we made a slow turn, pausing to snap a few photos of a sunken, scuttled ship, the Selma, one of just a few in the world ever built out of concrete (fewer still are intact and …