OSAGE BEACH, Mo.—The petite blonde bartender in ripped jean shorts bounced to each side of a square-shaped bar as women in bikinis and shirtless men lined up on a sweltering afternoon to order Bud Light, vodka and soda, and piles of nachos at this dockside retreat in the Lake of the Ozarks region. In a county designated a Covid hot spot, in a state with one of the lowest vaccination rates in the nation, and in a region where hospitals are nearing capacity as the Delta variant takes hold, Erin, a bartender at Backwater Jack’s, couldn’t be in a more...