As far as motion sickness, Dramamine (the less drowsy formula) works pretty well, although you have to take it an hour before travel, so if you are in doubt take it. Although, as mentioned before, most cruise ships are large enough that you won't feel any motion unless you have really choppy seas.
As far as luggage, I would pack one large suitcase plus a small duffel with your carry on items, such as valuables. For a five day cruise, you should only probably need a few sets of casual wear, a couple sets of evening wear, a formal outfit (if you want to eat in the main dining room), and swim wear.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)