What you will learn (or not learn as the case may be) in a cruise show is how to cope with surprises in performance. Anything can happen.
It will also teach you how to sustain a performance which is a skill just like singing. Some people cannot do more than 30-40 performances without getting so bored they start walking every performance. On a cruise you learn how to give the same show night after night after night after week after week after month... Oh, you know.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher