Yes, you will feel underdressed on formal night if you don't dress up at all. You could always not go to the main dining room and just go to the buffet on formal night. Depending on which cruiseline your going on, you could wear a short dressier cocktail dress or even a long dark skirt/velvet pants with a really flashy/dressy blouse and some nice jewellery. (on some cruislines things tend to be "not quite as formal" on formal night ie. dark suits for men instead of tuxes, outfits as above for women) Carnival, RC, Princess, Norwegian this would be o.k. but I would not recommend the above dress on a Crystal, QE2 cruise etc. Hope this helps, have fun.