Just a few hints about pesos... the first one is: get them and use them...not only is it fun, it is respectful of the local culture.
Some folks seem to think they may or may not need pesos while here. This is not like other tourist destinations in Mexico such as Cancun where you can use dollars or pesos. This is MEXICO. No way would you expect a Mexican traveler to use pesos in the US, so....
it is hard to get pesos before you arrive...no worries. No need to get pesos from your bank before you leave. You may want to get 50.00 usd worth of pesos at either airport - where you are flying from or in cancun - but beware - ATM's at the Cancun airport have a reputation for scanning your info and using it later.
You can make it all the way to Isla Mujeres without pesos, and when you arrive at the ferry, DIRECTLY across the street is an HSBC bank with 2 ATM's. not only is this the best place to get pesos, it also offers the best exchange rate of anywhere that is reasonable to get to.
You can use dollars in emergency situations here, but it is a matter of respecting the local culture by using pesos.