US Dollars are accepted everywhere, so there's no real need to get pesos for your visit.  It might be a little cheaper to pay with pesos, but usually not much, especially if you pay an exchange fee or surcharge.  

 

You will most likely be quoted in USD for souvenirs and services (jet ski, atv rentals, etc).  Remember to bring lots of small bills, as many vendors won't have change in USD.  Some vendors won't be able to make change at all.  Be prepared with lots of 1's, 5's, and 10's.  The convenience stores in the resorts will accept USD also, and can give you some change in MXN (pesos)

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Visa and Mastercard are widely accepted, and American Express and Discover are VERY RARELY accepted.  Your card may also impose a foreign transaction fee, so read the fine print before you go.  Smaller stores won't always be able to accept credit cards, and some places (including restaurants and resorts) may charge a 3% fee for some transactions. 

 

You must call the issuing bank to let them know you'll be using your credit card in Mexico.  Quite often they'll flag a foreign transaction as fraud if you don't alert them to your plans.  Most cards now have "chip technology" which is far harder to clone, but if you're concerned, cash is always safest.