But practically, what to bring? I suggest at least one of each of the following: a wool or fleece sweater/overshirt; a pair of long pants that are not too heavy (if they get wet, you don't want have to wait for days for them to dry); a bathing suit; some sort of foul-weather gear, even if its just a light raincoat. For the rest, assuming a cruise of a week or less (more than that and I recommend fInding a laundromat) bring 2-3 pairs each of shorts and summer shirts. A lifelong natural fibers freak, I have lately come to embrace the virtues of quick dry fabrics on the water. I like lightweight long sleeve shirts over short sleeves in all but the hottest weather because it's one less area to apply sunblock to. You probably won't want socks as often as you would at home, but it's a mistake not to bring any, because cold feet are the worst. Bring enough clean underwear for the duration. An extra hat is a good idea in case one goes over the side. You should always know where your towel is.
What about non-clothing, non food items? Know the bedding situation before you go and plan accordingly, but even if all bedding is provided, you might want to bring an extra pillow of your own. Pillows that live on a boat all summer inevitably take on certain pong- a cross, perhaps, between mildew and diesel fumes. Items for personal hygiene are critical. Ask yourself: what are the things that help you to feel cleaner when you can't take a proper bath? Bring those things. Any medicines you might require, you should obviously bring, although you may want to consider specialty items like Dramamine as well. For adults who drink, your preferred hangover remedy is probably a good idea as well. Bring books and magazines. Games that are very compact, like a deck of cards, are good to have.
As for luggage, multiple small bags are easier to manage than a single large one. Remember you might be sleeping with your bag(s) so rigid or poky luggage is to be avoided. Good luck.