One of the big pluses of cruising for North Americans is being able to visit foreign countries, but pay for shore excursions and onboard purchases in U.S. dollars. The system makes cruising abroad more affordable than land vacations, and gives passengers a break from constantly converting euro amounts to dollars in their heads—especially difficult when you're enjoying your third margarita. But if you venture onto a foreign-owned line(think P&O Cruises, Star Clippers, Fred Olsen, EasyCruise, and a few others), say goodbye to these perks—you'll find drinks, gifts, and tours bought onboard charged in euros or pounds.

Clarifying Confusing Onboard Currencies - SmarterTravel.com