I actually asked our PvP this question yesterday for our Alaska cruise, and even though our onboard folio states charges will be in USD, he said my booking made he in aus, we paid in AUD and any excursions I book will be in AUD, presuming of course I book them pre cruise, as is our intention.
So when I look under excursions on our cruise, I presume the prices (showing to me at least) will be AUD?