You don't have to dress up on the majority of ships as they have several dining rooms you could go to on what would be described as a 'formal' evening.
Most ships offer several options other than the main diningroom so not a problem. A few do Italian and Hawaii night meals .
I was wondering the same for the canaries cruise on the aurora, as i am not really looking forward to the formal night.
Thompson cruises would be best for you, they do not enforce any dress code, just like a workies club.