Khakis and a golf polo shirt are as dressy as I get on a vacation, and that has been fine on our last three cruises; no complaints from staff. We even have our photos taken on the formal nights and we have never been told that we were not dress correctly. I have seen some beautiful dresses and tuxes, and even a few kilts-very impressive and fancy, but like I said, this is my vacation, and since I HATE ties and suits for myself, I don't own either, and get along just fine in the dining room.
Have a great cruise!!