Why not check the bag with your clothes in it? At most, we might put a bathing suit in our carry-on, but the checked bags usually show up fairly quickly and even when they are a little late, the first night dress code is definitely casual.
I agree with the rolling backpack. That is what I use and it works great. I also have a feather weight nylon backpack that I use in ports. Just in case I see something I just have to buy.
The rolling one is just too ackward in ports. I have to let go and put it aside if I want to take a photo or do anything. Too easy for local pickpockets to snatch and go. Plus, I keep my port bag in the nylon one. That is a plastic case with sunscreen, bug repellant towelettes, tiny roll of TP, hand wipes, and a cheap disposable poncho in it. I pack it at home (actually, it stays packed), then put in the nylon backpack which goes in my suitcase. I hate spending time and energy on the cruise trying to decide what goes with me into ports.