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, RPLA is at km 8.5, so 2 km apart. Downtown to Puerto Juarez will be the 8.5, plus (I am guessing) another 2 or 3 km, perhaps a total of 11 or 12kms.

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 without air conditioning. They also seemed to have a bit of trouble getting it going. The one from Fat Tuesday was a smaller, open-air ferry that we thought was much nicer to be on.