It wouldn't bother me in the slightest, why cant they experience this wonderful new cruise ship and experience for themselves? If everyone thought that way, the cruise ships/holiday destinations would be empty, well not quite but...
I would most certainly not want to hear X, Y and Z about it before you go and you did right telling them that. How i see it is that you must have painted a wonderful picture for them to want to book it, which makes me then think, in the first place, if they are such avid cruise lovers and I sing from the roof tops of how absolutely wonderful the experience would be, am I paving the way for the possibility of them booking the self same thing ! In which case I would keep my gob shut... ( if i didn't want them to experience it, but as I say, it wouldnt bother me in the slightest )
I hope you have a FANNNNNNNTASTIC time, I have been in a few cruises and I know how fabulous they are.
Enjoy 