I was on the Carnival Victory for the 05-05-16 Journey concert. WOW, it was great, such a small venue, comfortable seating, good view and so close up that you can see the facial expressions of the band. the sound system is also good. I saw Journey last year at The Hollywood Bowl in Los Angeles and so much better in the small ship theater.
If a group you like is on a Carnival ship, go for it, so much better then a concert hall or outdoor theater. I will look to see who is on Carnival next year and I'll book in a heart beat of I like the band.
Only down side was no sales of concert t-shirts on board.