Just thank your lucky stars that you weren't aboard Cunard Princess following it's refit in Malta 1989. Passengers were greeted with banners proclaiming 'Cunard Line - 150 Years of Luxury and Innovation' only to discover raw sewage, holes where toilets should have been, faulty appliances etc etc. It was that bad that I allowed passengers to shower in my cabin on bridge deck. Needless to say Cunard compensated all passengers with a free cruise on the Princess or Countess and equally needless to say this did not go down terribly well. Unfortunately, Cunard has a consistent history of sailing at all costs following refits or in this case new launches regardless of the state of the vessel (and contrary to the advice of ships senior Hotel management). I would reccommend avoiding Cunard Vessels at these particular junctures!
for ladies, gowns OR dressy cocktail dresses are fine on formal/informal. Men should definitely wear a tux or you'll feel WAY out of place.
AND...ladies...you can probably get away w/ wearing a little black dress twice...for example, if you go to Todd English on one of the formal nights, no one will notice when you go to the brittania the next night. YOu can bring a colorful scarf or change your accessories. Don't sweat it, don't go crazy shopping. You'll bring more than you need.
And after if you hit the nightclubs, you'll want to change anyway.