Not long ago, when Andy Levine was aboard the ship Carnival Inspiration en route from Tampa to Nassau, Bahamas, he climbed onto the main stage and introduced Kid Rock to 1,000 cheering fans. All in a day's work. Levine's company, Sixthman, organizes concert cruises that allow music fans to get up close and personal with artists such as Kiss, 311, and Lynyrd Skynyrd. The floating music festivals last about three days and cost $500 to $2,000 per person. It has been just a decade since Levine, a former concert promoter, put down $750,000 on a lark to rent a cruise ship. It was a stab at giving devoted Sister Hazel fans what they wanted most: "a chill weekend of grilling and hanging out with the band." It worked, in large part because of fan loyalty: Vacationers on Sixthman cruises have a 60 percent likelihood of returning.