These are suggestions about things you might pack to go on a cruise. It assumes you're going somewhere sunny, but depending on your destination and how you like to travel, some of the items may not be useful to you, and there are inevitably additional items you'll want to bring along. But this should give you some ideas about how to pack. (Please keep in mind that we want this guide to be useful to people in general, so don't try to turn it into your own personal packing list.)
underwear, socks - They may not take up much space individually, but they can add up to fill a lot of luggage. Check to see if laundry service is available on the ship (and at what cost) to cut down the quantity you have to bring.
short-sleeved shirts - If you're sun-bound, these will be the staple of your day-time wardrobe. Depending on the formality of the cruise, you may want shirts with collars. If you're planning to go ashore, consider whether you want the shirts you wear to announce that you're a tourist.
comfortable slacks - (e.g. khakis)
formal wear - Most cruises treat at least one dinner of the voyage as a formal affair, and some encourage or expect you to dress up for dinner every night. If you'll be dressing up nightly, men may be able to get by with just an assortment of ties and perhaps a few shirts or a couple jackets.
comfortable walking shoes - Not critically important for on deck, but for excursions ashore make sure your feet will be prepared for whatever the terrain you'll find there.
windbreaker - Even in the tropics it can get rather cool on deck at night, especially in the ocean breezes.
swimsuit - It's ironic that you'll never touch most of the water you see, but medium- to large-sized cruise ships will almost always have a onboard swimming (or lounging) pool or several.
travel alarm clock - You're on vacation, so you may not mind about sleeping in. But if your room doesn't have a window, and especially if you've changed time zones, you may not see the daylight and sleep too late to get breakfast in the dining room.
empty tote bag - To come back with less in your bags than when you left is almost unheard of. So if you pack tightly on the way out, you'll need something to put your purchases and/or souvenirs in on the way home.
cash, traveller's checks, bank card
Stay healthy and safe
sunglasses, sun screen, hat - Even if you plan to spend your time on-deck in the shade, the sunlight reflected off the waves will still affect you. Granted, you probably want to return home with enough tan to prompt questions about your trip, but don't underestimate how a sunburn can spoil your trip... and your skin tone 10 or 20 years from now.
motion-sickness medicine/bracelet - You may not need this, but don't assume that you won't. It isn't a question of willpower or healthfulness; some people react poorly to the motion of a ship at sea, even if they don't notice it consciously. There are over-the-counter pills you can take as needed, prescription stick-on patches for ongoing treatment, and bracelets that (according to those who use them) help suppress the symptoms of motion sickness.
ID card/passport - Cruises frequently visit more than one country, and even if Immigration at these destinations doesn't require one from your country, it certainly doesn't hurt to have official documentation of your citizenship.
See and Do
camera, film/memory card
notepad and pen for journal
handheld GPS unit - Rather than bugging the crew, a GPS unit can tell where you are and how far until the next port.