No cash needed on board you just leave credit card imprint at pursers office when you get on board then you sign for all purcheses with your cabin # the night before you disembark you will recive an itemised list. This will then be charged to your Credit Card. At least thats the way it works on Holland America. I'm not sure but I think it's around $50 per day pre-authorized before you leave Port. So If you are going on a long Cruise make sure that you use a card that will let them authurise the total amount ie: 7 Days @ 50 350 X 2, If just two are travaling,=$700.00.
It should say some where in your embarkation pkg. How much they will need to authorise per day.
Bon Voyage
Happy Cruising