“Look,” my son said.

We were bobbing in the shadow of a pier on Isla Vieques in Puerto Rico. Wooden slats a few feet above our heads sheltered us from the tropical sun. Weather-beaten pilings disappeared beneath the water’s surface. It was cool there but barren—a human-made spot suitable only for a quick rest during our first foray into snorkeling.

Will pointed down. His eyes were wide behind his mask. He dipped his head underwater. I followed.

We entered another world. Above the water the pier was a dull structure of warped wood and chipped paint. Underwater it teemed with life—orange and yellow corals wrapping around the pilings, lush sea plants undulating in the current, darting schools of silvery fish. This narrow place