There are two different ferries to Isla Mujeres - one in the hotel zone behind Fat Tuesday and one in Puerto Juarez in the downtown area. The main difference is the frequency and the price. The Puerto Juarez ferry is every half hour, 70 pesos round trip. The Fat Tuesday is less frequent  and twice the price at 150 pesos round trip.

 Fat Tuesday is at km 6.5 in the hotel zone. Downtown to Puerto Juarez will be about half an hour from most places in the hotel zone depending on your location.  The bus is the cheapest way to get to either at a price of 6.5 pesos.  For Fat Tuesday's pier, any bus in the hotel zone will get you there.  For Puerto Juarez, you can either take a bus marked "Puerto Juarez" (not as frequent as the other buses), or take any bus downtown, get off at Bonampak (first street downtown, ask the driver) then take a taxi to the Gran Puerto for about 20 pesos.  Of course, you can always just take a taxi to the ferry dock, prices will vary from departure points.

There are actually two ferries from Puerto Juarez very close to one another. Make sure you go to the Gran Puerto ferry terminal. It's nicer, faster, and the same price. It's the first one you will pass and you will recognize it because it has a very tall lighthouse type structure with a restaurant on top.

 The ferry from Puerto Juarez was a larger, enclosed ferry with air conditioning. Drinks and snacks are available on the dock or on the ferry. The one from Fat Tuesday was a smaller, open-air ferry