We took a trip to Isla Mujeres through our resort. We were not really impressed with the beach or the city, but loved the snorkeling. The boat left from the beach and took you out into the clear area of the water, and you got off the boat in groups of about 10. The boat then left, which was a little scary, to get more people. Each group then swam and snorkeled, following their leader along the reef, rocks, and other formations.
We were out in the water for about an hour before our boat came back to get us. The swimming in choppy water was tiring, but we got the see more than other snorkeling trips that we have been on. I am not sure that I would recommend this trip for small children or older adults, just because of the fact that you did not have the option of getting back on a boat if tired. At one point, my husband got leg cramps, and our leader pulled him with the life ring until he could swim on his own.