It would be great if we could get all these suggestions into one big list as a MS Word document or Excel spreadsheet!
I, for one, always need more memory cards for my cameras. I never seem to have enough. I didn't want to take my laptop on my last cruise and I took over 5000 photos on my Mediterranean Cruise, filling up all my memory cards I brought and then some. Don't buy memory cards in a port unless you're able to check them and format them before you walk out the door. I had a counterfeit SD card sold to me in a port and the card would not work. Wound up buying extra cards on the ship for way more than they were worth.
If you bring your own bar of soap, you can make your own soap on a rope from any brand by using a drill bit and a 3 foot length of nylon cord. Works great for those of us with bad knees and backs when in those tiny cruise ship showers.