Probably one of the most important things you can do during your stay on Isla Mujeres is change your Dollars into Mexican Pesos. Many Americans, even some who have invested in Isla Mujeres real estate and spend a lot of time on Isla, still find it easier to spend dollars but there is a downside.
It is of course easier, and understandable that some people like to know what they are spending in Dollars, the problem with this is that you are generally going to be spending a lot more than if you use the local currency. I remember a visitor when referring to a certain journey in taxi said "It's so cheap only $3". Well the correct fare for that particular journey was 13 Pesos, which is 71 cents.
At the time of writing the exchange rate is just over 18 Pesos to the Dollar. However if you chose to pay in Dollars this is quite a bonus for those providing the service or goods becase they will ask for, as an example, 250 Pesos or $25.00. At the airport it is great for the Taxi service because they offer a taxi from the airport to Puerto Juarez - where you catch the ferry to cross to Isla Mujeres - at 700 Pesos or $70. Many Americans for ease and simplicity opt for Dollars. others because they have just arrived and haven't had a chance to change any money; although there is a cash machine just as you come out of customs.
All this dates back to a time when there were about 11 Pesos to the Dollar, so by getting Dollars they would be getting if you like a 10% tip, which is fair enough. However when you are talking about 18 Pesos to the Dollar the taxi, rather than receiving 700 Pesos they are receiving 1, 275 Pesos. Or put another way, you are paying $31 more, simply for the privilege of paying in Dollars. Because of money laundering laws there is a restriction in changing up cash Dollars to Pesos but you can draw out Pesos on your debit or credit card and generally you will have a less expensive holiday.