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Look in your brochure or type in "dress code" in the search for questions section and you will get the answer. Most cruise ships are the same with the exception of NCL and Azamara....I'm being lazy too....Neil's son Fala.
I have been on 3 Costa cruises. I have worn a Tux on one and Sport Coat the others. Costa isn't as formal as many say. They don't seem to enforce their dress code like other lines do.
Just my 2 cents.
Below is directly from Costa's website on dress code:
Daytime dress on a Costa cruise is casual such as light, cotton clothing or swimwear with cover up when indoors. Most every night is informal, feel free to dress comfortably in resort attire, jackets are not required. There will be two formal evenings where you may choose to either dress formally, elegantly in a suit for men and cocktail dress for the ladies, or more casually in elegant resort wear, as how you want to enjoy your night is up to you.
One of our informal nights is a Roman Bacchanal to which you may wish to wear a custom toga or accessorize your bedsheet/toga provided onboard.
For guests who are interested in dressing up more formally, you may also rent a tuxedo or dinner jacket by calling Cruise Line Formal Wear Inc prior to the cruise at 1 877 88 COSTA. Your formal wear will be delivered to your stateroom before you sail and picked up at the end of your cruise."
From what the above says yiou choose how formal you want to be.
If you do want to go to the Main Restaurant during formal nights, we have seen people being turned away as they were wearing jeans and a polo shirt.
Same as any other cruise ship. you can dress it up or dow.