I sentiment this post exactly! Use an Aussie credit card and ask them to charge in USD!!! If need be, even open a bank account that doesn't charge foreign transaction fees before departing just for this alone, it does make a big difference and choose one that has favourable exchange rates over others. From experience, I can recommend Citibank.
(I also use this account/card whenever I travel overseas, cruising or not). I have not been able to find a bank that's been able to challenge them in any of the above yet, even on exchange rates. If someone can prove me wrong, please do! The alternative would be to find a great exchange rate before boarding and purchase US dollars and settle the account via cash onboard before disembarkation.
You need a hell of a lot of cruising nights to reach that level though, geez! That's not the average person asking these sorts of questions here nor would they have the need to! They would have that knowledge. This information is of no use to the og poster and is an obvious way to flex your status of which nobody cares. 🙄