This will depend on the cruise and your personal spending habits. For ocean cruises, you should allow funds for Shore Excursions which are generally not included as well as social needs while cruising (drinks, spa, etc.). You may also need to allow for funds for the time that you go ashore. Most cruise lines use the US Dollar as their currency on board. You can charge your ship board expenses to your shipboard account and settle at the end of the cruise with credit card (easiest), travellers cheques or cash (some cruise lines also accept personal cheques). It is wise to have some cash (US Dollars) to look after small purchases off the ship, extra tipping and the like. ATM machines are generally not available or convenient for cruise passengers. On river cruises, most shore excursions are included and you will need funds for extras (e.g. drinks), expenditures on shore, and possibly some optional excursions.