Most cruise ships take credit cards or cash - some take small denominations of US$ travellers cheques. Debit cards are not taken.
If you have not already visited - suggest you go to
http://www.cruisecritic.com - half way down the front page there is a little box with 'Read Members Reviews' - find your ship and see what previous travellers have had to say.
Then at the top there is a link to Boards - you have to sign up to read the section on your cruise line [easy to do and free] - otherwise there is a section called Ports of Call so you can read what to do in each port you visit.
A mine of valuable information. You did not say who you were cruising with but hope you enjoy!