I just came back from a cruise, and I found that you really only needed about 3 or 4 outfits a day:
1 for the morning excursion
1 for dinner (usually swim before dinner)
1 for after dinner shows
1 for night clubs
I packed by outfit instead of just 6 pairs of shorts, 3 pairs of shoes, etc... and i didn't have too much or too little. I brought 7 pairs of shoes, all of which I wore, and a whole suitcase for "cosmetics" (but I used all of them). But Just Plain Me is right!, It's better to overpack than underpack!!