Have cruised twice on the Celebration, shirt & tie without jacket would be totally accepable, maybe on Formal Evenings(usually 2 per cruise) jacket would be advisable, or
put on a dinner jacket and black tie, or eat in the Lido, where there is no dress code(Food just as good) Bill.
Yes you'll be perfectly fine wearing a shirt & tie without a jacket. If you want to look smart, try not to pair it with jeans & trainers though!
It should be ok.... on formal nights I have taken my jacket off without a problem.... and it can be pretty darn warm with a jacket on.
It depends on the sort of night it is. On full dress night you must wear a tie and jacket but on formal nights a jacket without tie is ok.