We flew into Cancun last month and used the following website to book our transportation which was recommended on this board. We were very happy.
http://www.entertainment-plus.net/tour_transport.cfm
We booked a private car to Puerto Morelas for $79 round trip; and the driver was there waiting for us...in fact he found us!! And they picked us up on time to return. We were at the new El Cid Riveria Maya. There were 2 of us and it was wonderful to have only ourselves to drop off.
We also asked our driver Patrick if he knew of anywhere to get a cell phone for use while we were there. He said he does it on his own and offered to bring us one the next evening at our resort. He did and he only charged us for the minutes we wanted him to load on it. We gave him a generous tip for doing it. He also gave us a charger to use and helped us learn how to use it. It worked great. Then we just returned it to the driver that we had returning. While we were riding to the airport he called to see if it worked ok for us. What service. We were extremely happy.
Now we need to research getting a cell phone in Mazatlan too.
Judy