The son of a son of a sailor proved his mettle during a recent fishing expedition off the coast of Nantucket. Singer Jimmy Buffett, whose catalog of favorites includes many a nautical reference, snagged a 350-lb., 74-inch-long tuna on a day out at sea. The musician was enjoying a day off between two shows at Jones Beach on his summer home of Long Island, NY, when he was surprised by what he found at the end of his line. The catch took Buffett an hour or so to reel in. No word on whether the songsmith enjoyed the tuna with a bottle of LandShark beer…