I emailed concierge about currency and this is their response:
Currency Exchange - Disney Magic
Currency exchange is available at Guest Services. The rate of exchange can fluctuate; therefore, guests should check with Guest Services.
There is no limit to the amount of money that can be exchanged onboard.
The following types of currency can be exchanged to US Dollars for no fee, with the exception of the Transatlantic, Mediterranean and Northern Europe sailings:
During the Transatlantic and Mediterranean sailings, the currencies listed below can be exchanged to Euros.
During the Northern European sailings, the currencies listed below can be exchanged to Euros, Russian Rouble, Swedish Krona, Danish Krone and UK Pounds.
Australia Dollar
Brazil Real
Canada Dollar
Cayman Dollar
Denmark Kroner
England Pound
European Union Euro
Hong Kong Dollar
Japan Yen
Mexico Peso
Norway Kroner
Scotland Pound
Sweden Kroner
Switzerland Franc
United States Dollar
If a guest exchanged US Dollars to Euros, Russian Rouble, Swedish Krona, Danish Krona or UK Pounds on the ship, the guest can exchange these funds back to US Dollars as long as they provide the onboard receipt.
I asked about making payments on our stateroom account using other currencies, and they said it could only be USD. I thought others said they made Euro payments, but to be safe I will not plan on using extra foreign currencies for that.
Based on this, I think I will only bring UK Pounds and Euros, since I can use them when I travel for work in the future. The other currencies, which I may never use again, I will buy on board so I can exchange back what I don't use. I figure I won't need much so the exchange fee won't be huge and I have to pay an exchange fee at the bank anyway. Plus I won't be stuck with unique foreign currencies if (yikes!) we can't make a stop at one of the ports.