Look in your brochure or type in "dress code" in the search for questions section and you will get the answer. Most cruise ships are the same with the exception of NCL and Azamara....I'm being lazy too....Neil's son Fala.
You can never be overdressed on cunard. You could wear an evening dress every night if you wanted to. there are no casual nights. Men where jacket and tie on regular nights and evening wear on formal nights. On a 8 night cruise there are 3 formal nights. Women can wear a different dress every night if they have it.
You'll get plenty of opinions.....so lets start with what Cunard says:
The general atmosphere onboard is elegant, but relaxed. Although shorts and swimsuits may not be worn in the ships main restaurants, casual attire (including shorts) is welcome during the day at the Kings Court on the QM2 and the Lido Cafe on the QE2. Swimsuits, brief shorts and exercise clothing should be reserved for poolside lounging, deck activities and the Spa Clubs.
Your Cunard Voyage Guide, p.14.
Formal: dark suit/white shirt & dark tie. - You will be in the minority as most men wear tuxedos but your choice is also accepted.
Casual: khakis & dress shirt or polo shirt. - Cunard does not haVe casual nights but only elegant casual ones. A jacket is required, a tie is not even though a number of men wear a tie on these nights, too.