Some ships are more formal than others but it's a common misconception that you have to wear a tuxedo or ballgown to dinner every night. Each cruise line will have information on their dress codes either in the brochure or on their websites, so when you're choosing your cruise just make sure you have a look so you know what to expect when you arrive onboard.
What to wear during the day
During the day it's generally pretty casual so you can wear whatever you like, but be mindful of where you are sailing to and any potential excursions or tours that you'll be doing. Alaska, Norway and the Baltics can have very changeable weather so layers are a good idea and take a warm, waterproof jacket with you too. If you are visiting any religious buildings like churches, mosques or temples, you may be required to cover certain areas such as your arms and legs. You may end up doing lots of walking when you are off the ship too, so make sure you have comfortable shoes!
What to wear in the evening
Most cruise lines will have a couple of 'formal' nights on each cruise where you would be expected to dress up in smart evening wear, but they aren't mandatory so if you don't want to get involved there will be other restaurants and bars for you to go to with a casual dress code. If you want to be casual every night, take a look at Norwegian Cruise Lines and Carnival Cruises as they have very relaxed dress codes all of the time.
Top TipsBe realistic about what you will actually need and so that you don't over-pack, and don't be afraid to wear something more than once; it's not unusual to see women wearing the same dress or top several times!