I’m strolling towards the sunset on a walkway next to a white-sand beach that’s lined with coconut trees and open-air bars. People are dotted along the coastline, walking, swimming and sunbathing. There’s a soundtrack of gently lapping water. Just off the northern coast of the Yucatán Peninsula in southeastern Mexico, Holbox island is about 26 miles long and a mile wide. Its population hovers around 3,000, depending on the season. Life here appe…