The Cunard website says little--"dress shirt and trousers" with the aspirational statement that the dress should be appropriate to a "stylish restaurant" or theater on a special evening.
I just saw a photo, probably from Cunard, of two couples dressed in smart attire. The two men were wearing a collared shirt and a sports coat, but their trousers looked denim.
I also just read a travel agency website addressing evening attire on Cunard--it talked about slacks and a sweater or a collared shirt and sports coat.
Look, this is my first Cunard experience, and I knew before signing up that Cunard was a formal cruise line. I am not debating what the dress code should be--again I think a collared sweater is more dressy than a collared shirt without a sports coat. Whatever the rules are, I will comply with. But I have limited luggage space for evening clothing. To those who have been on winter Cunard sailings, I am just trying to find out whether the four-word website statement of "dress shirt and trousers" is literally the rule or whether it is somewhat a guide and aspirational.