The island nation of Palau is home to some of the world’s most pristine coral reefs. Here, two pink anemonefish, a kind of clownfish, swim above an anemone that’s nestled within a patch of lettuce coral. PALAU, Micronesia — The ride out to sea was, in a word, unpleasant. The coastal waters of Palau, an island nation in the western Pacific, are typically calm and electric blue, like what you might see on a screensaver or postcard. The country has put in place some of the world’s most rigorous protections to keep its marine environment pristine. But on this particular morning in July, a...