Not formal it is suggested that ladies wear skirts or trousers complemented by a sweater or blouse. Gents should wear trousers with a collared sports shirt or sweater. Hope this helps.
The following is an extract from the Princess Cruise Answer booklet: "Evening attire falls into two categories - formal or smart casual. If dress is smart casual, passengers are asked to dress as they would for a fine restaurant at home. Appropriate attire includes skirts/dresses, trousers and blouses for ladies and trousers and open-neck shirts for men." Hope this helps.