No. There are usually 2 formal, 2 semiformal and 3 smart casual nights per week. First and last nights are casual. The following descriptions are from P&O's website
This is the opportunity for ladies to wear formal evening wear such as a ball gown, trouser suit or a cocktail dress. For men, a dinner jacket or tuxedo are the norm, otherwise a dark suit and tie or even a kilt and jacket is fine.
Ladies can wear anything from tailored trousers to smart separates or an elegant dress, while men can wear a jacket and tie. However, smart shirts that do not require a tie are also acceptable when worn with a linen jacket or blazer.
Stylish resort or leisurewear is ideal, for example casual separates, dresses or smart jeans for ladies and openneck shirts, polo shirts and trousers or smart jeans (not shorts) for men. Men are not required to wear jackets on Smart Casual nights. We would request that sportswear, (e.g. trainers, tracksuits and football shirts) are not worn in the evenings.
No, casual nights are just that, casual.. There is, however, nothing to stop you wearing a tie every night and people will comment how well dressed you look. That is not said in a facetious way either .... Nei.