Athough I have never cruised with Cunard, the dress code for daywear may be similar to P&O.
Swimwear is for around the pool and nowhere else. If the pool area has a bar and you do not use the bar stewards to get your drinks, you can go up and get drinks in your swimwear, but if you go to an inside bar area appropriate dress must be worn, same for dining rooms. Yes, you still get some who try it on, but these normally get told.
Somewhere at the back of the Cunard brochure you should find dress rules.
Hope that helps.