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It depends on the ship. If you don't want to dress up try Ocean Villag.
No ship requires dinner jackets and on all a lounge suit is perfectly adequate. The most formal is probably Cunard and is great for those who like formal nights but even they do not require dj's. At the other extreme Ocean Village definitely does not but this is more appropriate for the younger set and those who do not want any formality and perhaps the overall standards suffer as a result. Most important thing is to be yourself, enjoy yourself and choose the type of cruise that meets your requirements.