The inaugural Mountain Song at Sea, a bluegrass festival on the ocean, just added the Punch Brothers to a line-up that already includes the David Grisman Sextet, Del McCoury Band, Steep Canyon Rangers, Tim O’Brien and Bryan Sutton, the Kruger Brothers, Peter Rowan, Larry Keel and Natural Bridge, The Travelin’ McCourys, Shannon Whitworth, Della Mae and Town Mountain. The cruise…