Jeans are fine during the day... as is sports clothes / bikini etc
P&Os dress code is shirts must be warn in restaurants / food areas at all times (& i think shoes (flip flops) too) In the evening there is a dress code, casual, informal, formal (on some long cruises black and white)
Jeans are not ok for informal +. On formal nights men are expected to be in a suit with tie / bow tie etc, and yes ladies do and will where ball gown type formal dresses, but so long as you are in a smartish dress, not a dress youd throw over your bikini on the beach you will be fine
Anything goes basically, yes there are some who wear full length ball gowns and one wonders how they fit it all in their cases.........but most people wear knee length cocktail dresses for dinner or trousers/skirts and spangly tops, men in DJ's. Some people never go to eat in the formal restaurant, they prefer the relative informality of the self service restaurants
During the day anything is acceptable - although again you will see folk in yachting caps and nautically themed outfits - I often wonder if they wear them at home or just when they go away on a cruise!