Visa is the most widely accepted card. From a hard lesson learned 20 years ago (mom had wallet stolen in Spain, they both carried same card, thus had no card for 4 weeks) hubby & I always carry different cards. One is a Visa and the other is either Discover or AmEx. Traveling alone this year I carried 1 card and locked the other in the safe.
We have used our Discover Card to cover our Sail and Sign card but learned the hard way that some of the stores in port don't accept it as it was the only one we took along and then couldn't buy what we wanted to get.
Now my husband and I also take different cards with us....one of each...that way we aren't caught empty handed if something happens. I usually take my Visa into ports and some cash...he takes travelers checks so we are covered in case we want to buy anything. Just watch using your credit card in a non-recommended place.
As a precaution, I contacted all three of my major credit card companies to let them know we were going on vacation to the Caribbean - Discover replied that they did not think the card would be accepted in most areas. Just thought I would pass this info along.