Check on the Visa site to see where Visa cards can be used in Puerto Rico .
Click here http://visa.via.infonow.net/locator/global/jsp/SearchPage.jsp
I have ordered one for our holiday in Florida after being used to the Amex one , will be interesting to see how useable it will be .

I've just had a look at that site and would suggest you don't take it as gospel that it shows all available. Checking out Visa ATMs in Tunisia, an area I know well, there is only one listed in Sousse and none in Port El Kantaoui. There are, in fact, ATMs accepting both Visa and Mastercard every few hundred yards in both places, (many of which have had my custom over the years.)