The last time I came to St. John, one of the islands that makes up the United States Virgin Islands, was eight years ago. I was there to leave my father’s ashes behind. It was his favorite place, and, in memory, at least, it was mine, too. He visited before my birth, and then many times after. He loved the solitude, he told me, the permission to sit on the beach with nothing but a book, a mask, and a set of fins. He loved to snorkel, a whole world unfolding beneath the turquoise water, always a surprise, even when you had seen it before. For many years, we went to the island as a family, my father — my ever-adventurous water companion — chasing turtles and rays and whatever great adventure the water promised next.  Sometimes, it’s hard to feel the same way about a place that looms large in memory. But St. John, I recently discovered when I recently returned, doesn’t suffer that indignity. In 1956, Laurance Rockefeller placed 60 percent of the island in trust with the National Park Service. Since that time, the majority of the island has remained pristine, with no buildings sullying the coastline. On any number of beaches, you can look up and see not a single thing be...