Well, depending if you can bring it all in your suitcase, or buy some things on board.
An adaptor. Depending what country you're from, you may have different electrical cords.
Toothpicks and dental floss.
A Laptop case.
A BATTERY POWERED alarm clock
Magnets. That way you can easily put photos and lists up on your cabin wall
Maybe even a raincoat - depending where you will be
Vitamins - Vitamin C in particular. Being on a boat, colds and flu can spread quickly. You want to try to be as immune as possible.
Sea sickness tablets - Never know if you'll need them
Hooks. Hooks you can hang over doors or stick to walls. They will be your life saver since room will be limited.