The only place you'll be able to use cash on board is for tips. And even there, the only place you'll need cash on board is room service (and luggage handling, though you are off the ship when you hand over that cash). We tipped our servers and room steward in cash, but you can charge those if you wish.
If you are standing next to the bar really craving a beer with $100 in your pocket, you won't get a beer with the $100. (OK, maybe the bartender would sell you a $100 beer, but he isn't supposed to).