The method by which passengers pay for things on a cruise often depends on the type of ship and the amenities already included in the price of the ticket. Many larger cruise ships work like all-inclusive resorts, with almost all typical expenses (food, lodging, entertainment, shore excursions, etc.) included in the original ticket price. Others operate more like luxury hotels, with individual accounts opened for each passenger so they can charge things to their room.
While a charge account on a cruise ship may allow passengers to charge nominal expenses and pay the total with a credit card, it doesn't mean a cruise is a completely cash-less environment. Passengers who wish to gamble on board, for instance, must provide their own cash to purchase gambling chips or tokens. It is not unusual to find at least one Automated Teller Machine (ATM) on board a large cruise ship with casino-type gambling.
Depending on the cruise ship's food and beverage policies, passengers may also have to pay for drinks on a cruise, especially alcoholic beverages. Some cruise lines offer a separate beverage plan that allows passengers to consume unlimited amounts of non-alcoholic drinks like soft drinks, tea, coffee, and water for a nominal surcharge. If this plan is not available, then passengers may need a supply of cash to pay for non-complimentary beverages. Tipping the bartender or wait staff may also require some cash if the passenger doesn't plan on tipping all of the service staff at the end of the cruise.
Shore excursions may be included in the original ticket price, but this doesn't mean all of the expenses passengers may incur are covered. Cruise packages typically cover things that are provided on the ship, not the activities the cruiser may take part in in the countries the cruise liner may visit. Transportation not arranged by the cruise line for an organized tour may not be included in the ticket price. Passengers may be able to purchase food or gift items with their own currency, but they may also have to make arrangements to exchange their cash for local currency.
In general, passengers on a cruise ship should plan to carry some cash with them on the trip, even though most of the nominal expenses can be charged to an individual guest account and paid with a credit card at the end of the voyage. If a passenger plans on spending significant time in a port city, having a supply of cash to cover food, entertainment, and travel expenses may be especially important.