It may be difficult to find an ATM if you are in Tulum without a car.  The airport ATM gives pesos, but has an exorbitant commision fee,  without stating what the exchange rate will be.  The cambio at the airport has no commission, but a poor exchange rate.  Ana y Jose centrally located on the hotel strip has two ATM machines side by side at the entrance to Reception on the jungle side of the road.  One ATM gives pesos, and the other gives US $$.  Very easy and safe to use.