I agree with PapaBill about the over-the-hill entertainers doing the cruiselines as their farewell tour. The last cruise we took a comedian/singer who've I've never heard of before rambled on for 15 minutes on how this was his last gig and he was retiring after the cruise. First he wasn't a very good singer but he let us know all the shows he's done for 25 years, second he wasn't very funny. Put the two together and he was boring.
I do like the high energy, fast paced, costume changing, medley productions. I think they are entertaining and I always try to make these shows. What I don't like are the off-color comedians who think dropping the F word 25 times a joke is funny.
I would like to see theatre, i.e. plays. I would like to see popular (if past their prime) musical groups. I know you won't see the Rolling Stones on the Brilliance of the Seas, but there are plenty of groups who tour around the country now doing smaller venues which I would like to see booked.