Buy some water to have inside your cabin, caz' inside the ship the water they have for free is desalt, wich tasty really bed or the water they sell, that is very expensive!
You don't know when they will serv you food, so have some food in your cabin that will not spoil easilly. so, cookies are the best meal for this ocasion.
3. small luggage
You'll gonna have too little space for your staff, so take a small luggage, even those, you'll buy some more while you are out.