Formal and casual are easy to define... I think informal should be called semi-formal. On the Mercury last January, for informal nights, I wore either a knee length dressy-dress or velvet pants with a beaded top. Most men that I saw wore a jacket and tie, and removed the jacket for dinner. Some just wore a shirt and tie, no jacket. If you're traveling with a woman, I'd suggest that you dress to complement her outfit. If she's wearing something a little more dressy (like I prefer to do) on those nights, then wear the jacket. If she's more of a casual dresser... go for just the tie or wear nice slacks and one of those collarless men's dress shirts so you can't wear a tie. lol My dh doesn't cruise... but I take our two teenage sons. This is what they do... Formal night: dark suit, white shirt, tie, dress shoes. Informal: suit pants, shirt (color), tie, dress shoes. Casual: chinos/khakis, polo shirt, sneakers. I did not see jeans in the dining room... not once.