We normally budget $100/per port for spending money, cab fares, drinks, tips, shopping, whatever. We always take travelers checks and cash one the night before we go ashore. Of course, we have already booked our excursions before hand. On board, all you will need is your sign and sail card. We usually do not eat on shore unless it's a snack because the ship food is wonderful, however, in Jamaica the jerk chicken and pork is wonderful. In Jamaica and Mexico and even some extent in Grand Cayman, you can barter. The only cash we use on board is to tip the waiters or bartenders for drinks and the only reason we tip on top of what is added to the bill is so we know they will get the money and not have to share it. I also appreciate waiters and bartenders that remembers what I have to drink after two or three nights, considering they see thousands of people. Plus we tip the cabin steward and dinner waiter a little extra at the end of the cruise. You do not have to tip extra, but we just appreciate the excellent care. If you are planning on any large purchases on your credit cards over seas, be sure to let your card holder know you are going out of the country. Enjoy your cruise!
Your correct in that one board you can just use your sign and sail card, so you really don't need cash. However, its always nice to have at least a few bucks for quick tips (say for the piano bar performer or the like). Credit cards work in the ports too. They are not going to loose out on your money by only taking cash.
The only reason I bring cash is for the casino. If you are gamblers, bring cash (in travelers checks) for that. You can get money from your credit card, but they charge 3%. The casino already has the better of you, why give them an extra 3%?.
There is a safe in your cabin. You can put whatever cash/travler's checks that you don't need for the day in it and only carry what you need.
On my 7 day cruise with DH last week I took far too much cash. We had about $400 in 20's, of which we used max $50. We had 100 $1's and once again I wished I had another 50 (for tipping and spending in port- you want SMALL BILLS). The rest was on the ship card. We spent $780 in 7 days on the shipboard account. It adds up!!!!
Just before I go on a cruise I stop at a bank and pick up a supply of those ever so hard to get $2 bills-they work like magic when it comes to tipping for that ever so special services that crewpersons give-tell them its for good luck and make sure they see it is a $2-you will be noted on board wherever you go!