Inside the Cancun Airport Terminal, you will see men with the ADO logo on their red shirts and from them you can purchase a ticket to El Centro (Cancun town centre). After you purchase the tickets (86 pesos per person as of June 2019) proceed to the bus outside the terminal which is clearly marked ADO/CENTRO (there is another bus that goes to Playa del Carmen but it is marked ADO/PLAYA DEL CARMEN). Your luggage will be stored under the bus. If you miss the ticket sellers the bus driver can also sell you a ticket. The bus leaves every half hour, takes approximately 30 minutes and goes directly to the downtown bus terminal with one short stop on the street in front of Plaza de Los Americas.
At the bus station, reclaim your luggage and go through the bus station to the exit (salida). On the street outside the terminal there is a bank of taxis waiting to take you to your destination (Gran Puerto). Usually the cost is 50 pesos for the taxi (less than $4 U.S.) but it is best to double-check this price prior to entering. The ride to Gran Puerto will take approximately 10 minutes.
At Gran Puerto proceed to the UltraMar ticket desk across from the OXXO convenience store to purchase your tickets (a round trip ticket costs 300 pesos per person) and then proceed to the waiting area next to the pier. Once the people on the incoming boat have completed the de-boarding process the rope will be opened for you to embark and an UltraMar employee will collect your ticket at that point.
Be aware that at Gran Puerto outside the UltraMar office there are people, wearing UltraMar uniforms, trying to set up appointments to sell timeshares. They will try to engage you in conversation when you are walking to the boat. One of the ways they do this is to ask you if you have your ticket and then ask to see it. They may tell you that the boat is not leaving for 15 minutes even if it is ready to board. Unless you don’t mind missing the boat, do not give your ticket to anyone except the Ultramar employee that asks for it prior to boarding.