Packing for a cruise can be a bit daunting. You will need casual wear for days and dressy or formal wear for evenings. Most cruise lines have relaxed their dress code but still require certain attire for the main dining room and special events.
When I first began cruising this was a big concern for me as my style is much more casual. I don't own any formal wear and I only take one small carry-on bag when I travel, so I had to do some research on what would be appropriate.
What I discovered is that casual resort wear is perfect for days at sea or in port and in the evening a nice cocktail dress or dressy slacks and top are more than acceptable. You will still see people dressed in gowns and tuxedos but I haven't been on a ship that requires it. Also, there are so many different dining options that don't have a specific dress code and you can always order room service and have dinner in your room.
My standard cruise packing consists of - 2 or 3 bottoms usually black slacks/skirts/capris, 5 or 6 print tops, 1 or 2 dresses, scarf, lightweight jacket or sweater, tennis shoes, dressy sandals or flats. I make sure everything is color coordinated so I can mix and match to make several different outfits.
I try not to overthink it. I just keep it casual and comfortable and it always works out great.