We cruised on the NCL Jewel last year and have also cruised on her previously. On our first cruise we charged everything to a credit card however on the last cruise we wanted to work within a particular budget and whilst booking in were able to pay an amount of cash upfront which then became our 'spend' whilst on board. The pursers office was fantastic and would happily check your balance (i.e. amount left to spend) at any time throughout the cruise.
No cruise ships take hard currency. Everything is charged to your 'room'. Credit cards only. Not Debit, Switch, Maestro or any other Mickey Mouse cards. There is no facility for handling cash on cruise ships.
At the end of all the cruises I have been we always settle our bill at the end of each cruise with cash, we have to register our credit card, but do not pay our bill with it. But all cruises you have to use your cruise card for all purchases in shops.
On cruise ships you normally need cash for the casino, which currency will this be?