Inside Zac Brown’s Free-Diving, Spearfishing, Fan-Boying Visit to ‘Survivor’

The boat is small and gray, cutting a lonely wake through the turquoise water off Fiji’s Mana Island. At the helm is a man who looks more like a local fisherman who hasn’t seen a razor or a hotel shower in a week than the multi-platinum country-music stadium act that he is: Zac Brown . Tattooed on his right shoulder: Bob Marley.

On his left, improbably: Teddy Roosevelt. He’s drawn to Roosevelt’s idea of the man in the arena, the one who acts while others watch. “There are people who are going to hate on whatever you do,” Brown tells me later. On the floor behind him are two four-foot dogtooth tuna, silver bellies catching the sun above neat slits, speared by his hand about an hour ago.

To get those fish, Brown, gripping a custom-built Alemanni speargun, plunged 80 feet into some of the sharkiest waters on earth. He had 90 seconds to take aim, fire, and ascend with the bleeding carcass, one at a time, back up through the water column before a shark could swipe it. It...