The cruise ship web site will tell you what you can or can not wear. Most of the cruises I have been on do not allow jeans on formal nights. We do not attend formal nights so eat in one of the other restaurants on those nights. Never been a problem as on Hollan American Line they serve the same dinners in the buffet that you can order in the main dining room. Now so many of the ships have so many restaruants to choose from you will not have a problem.