In 2009, Jeff Denholm was making a living as an adventure athlete, competing in stand-up paddleboard races and riding giant waves at Mavericks, the famous surf break near his home in Santa Cruz, Calif. Denholm was sponsored by Patagonia Inc., but to generate extra cash—“Adventure athletes don’t make a ton of money,” he says—he had a side gig leasing a fire truck to state and county crews that had run out of equipment ­battling wildfires.