Divers descending into azure waters far off the Texas coast dip below a horizon dotted with oil and gas platforms into an otherworldly landscape of undersea mountains crusted with yellow, orange and pink coral as far as the eye can see. Some of the world's healthiest coral reefs can be found in the Gulf of Mexico, about 100 miles (160 kilometers) off the Texas coast. Sheltered in a deep, cool habitat far from shore,...