It is quite acceptable to wear short cocktail dress or a smart top and trousers .If you do not like dressing up you dont have to eat in the dining room there are alternative dinig venues. It is fun tho to dress up.On one of the evenings some ladies wear little coktail hats usually little feather things [ t j hughes in Widnes sell them] I went on the transatlantic crossing last year , spent 5 nights in New York and then sailed back . I am doing more or less the same trip again this year and will be in New York for indepence day . It is a wonderful experience,and i much prefer a transatlantic crossing to a cruise . If you want to know more then ask me.
Knee lengh dress will be fine. there were quite a mix of dress when we went long or short dress will not be out of plac.
You can wear knee lenght cocttail dresses, or either nice slack.