Shark humor has its time and place, but not when I'm snorkeling somewhere called Shark Bay. At the Heron Island Research Station, a laboratory on the teardrop-shaped atoll 45 miles (72 km) off Australia's east coast, the suntanned, chirpy station manager gives a parting wave to the three students who are taking me out for my first look at the legendary corals of the Great Barrier Reef. "Just don't get eaten, will you?" she says. Ha-ha. Happily, there are no sharks in Shark Bay that morning; in fact, there's not a whole lot of anything. As I follow the students' snorkels, we pass over circular beds of brown, monochromatic coral and empty expanses of rippled sand. A handful of small, glimmering fish hover in the water column, but they're the only life we see during an hour-long swim. Where are the schools of coral trout? The famed Maori wrasse? Wading back to shore, one of the students shrugs: "Sorry there wasn't more."...