You didn't specify where you will be cruising to. I presume it would be someplace warm.
I would pack the things that you normally would pack when spending a day at the pool or beach; swimwear, sunscreen, hats, sunglasses, sandals.
I would pack shorts and t-shirts to wear in the daytime, and then long pants and nicer shirts to wear in the afternoons and evenings. Maybe pack a couple of hoodies and some long sweat pants to wear on a chilly morning. Then, of course, there is formal night, which you'll want to take into consideration if you're into that sort of thing.
Don't over-pack though. what I like to do is mix and match, where I can wear the same shorts or slacks with more than one shirt or viceversa.
ill give points