I remember in 2009 when we went on a Med cruise on NCL. One day we got back from a long day on shore in Athens. It was cool weather (February) and I wore jeans on shore.
My wife and I were both tired and just wanted to eat and then go back to the cabin and crash. We didn't change clothes and went to the Italian restaurant on board. I have to admit I felt a little "strange" eating in jeans but it was perfectly within the dress guidelines. We weren't wearing torn or worn jeans but there was still that feeling of being out of place. I also have to admit that the feeling only lasted about five minutes.