Should I use dollars or pesos?
I DO NOT recommend converting dollars to pesos at the airport in the states before you get here...you will definitely get a lower exchange rate (i.e.: 9 pesos to the dollar vs. 10.7 pesos to the dollar). I do recommend changing some dollars to pesos once on the island however, as you will get a better exchange rate that way. Although everyone will take dollars, the exchange rate in most places is at a set rate, and it will always be higher then the banks and change house rates. For example, they will have a set rate of 10.5, when the real exchange rate is 10.35. On $100 that costs you an extra $1.50. While it may only be a few dollars here and there, that does add up over the week. So, it is always best to pay for your meals in pesos, hotel in pesos, and souvenirs in pesos. Most dive shops I know of, will give you the real exchange rate for whatever it is that day on the island.
There are change houses ALL over the place and they are almost all owned by the same person, so no need to shop around for a change house with the best rate...they are virtually all the same. The banks typically offer a slightly better rate than the change houses, but be prepared to stand in line forever. Still, the exchange rate is usually a point or two lower than the actual market rate on Wall Street.
Another tip is to purchase something with dollars at Mega or Chedraui (grocery stores) as they have the best exchange rates. They may have a limit as to how much they will exchange for you however.