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