To add to what Sr.
@Rich One wrote:
There is less acceptance of credit cards by retailers (I understand this is because of the time it takes for the retailer to get the money). The exception to this is online purchases, e.g. bus / ferry / plane tickets, where debit cards are often not accepted. I've also seen that foreign debit cards are treated as credit cards for local transactions (don't know how this happens, but it does).
People send themselves money via Western Union and Moneygram, paid out in Pesos. Some cuevas can also receive funds and pay out in either USD or Pesos. Lots of info in the forum, entire threads, even. Always have a backup method of getting cash.