We did "around South America" on the QM2 in 2009. (According to DH: best trip ever!!!)

Never spent a penny of local currency. Booked tours through Cunard, which included lovely luncheons (great food with wine/beer).

We didn't venture out on our own and felt safe and happy under Cunard's wing.

Bought a couple trinkets near the dock in Chile and paid in dollars. Bought paper flowers in Mexico (at a market further from the ship) and paid in dollars.

Most "nice" shops and restaurants, world-wide, take plastic. Most vendors near ships (full of Americans) take dollars.

I think your currency needs will depend on what you like to do, but I can tell you: it doesn't need to be complicated.

Happy Sailing! Jimmybean