To enter Mexico you will need to present the following: A valid PASSPORT, Mexican Tourist Card and Customs Form which you will be given at the airport when you check in or on the plane.
GETTING TO ISLA
The Ultra Mar ferry, located at Gran Puerto, leaves every half hour from 6:00 am to 8:30 pm and every hour from 9:30 pm to 12:30 am. Ferry ticket costs $35 pesos per person one way. (ferry schedule)
There are several ways to get to the ferry. Whichever way you choose, once in Cancun, head to Puerto Juarez, which is both an area of town as well as the name of one of the docks. The other dock is named Gran Puerto.
Airport Shuttles: At Cancun International Airport, the airport shuttles (located outside of baggage claim) are very reliable and reasonably priced (usually $12 USD per person). The shuttles will escort you to Puerto Juarez. A shared shuttle will be much cheaper but will take longer. Puerto Juarez is the last stop, and you will get a tour of the Cancun Hotel Zone as other passengers will be dropped off before you. Travel time ranges from 60-90 minutes. Cost: $12 US per person (or more) and can be paid in US dollars.
Public Taxis: An alternative to a shuttle is a quick cab ride. All the cabs leaving Cancun International Airport belong to one company, which means they are safe and have fixed prices. Just tell the driver where you’re headed and how many people are in your party. A private taxi will cost approximately $57 USD to get to the ferry. Travel time is about 30 minutes. This is the easiest and fastest way to get to the ferry.
Private Van: You can also arrange for a private van to take you and your group directly to the ferry dock.
Buses: When you leave the terminal at Cancun International Airport, walk directly to your right and follow the sidewalk along the departures section until you come to the end. There you’ll find comfortable, air conditioned buses run by ADO. Costs are very low. The bus will drop you off at the bus station in downtown Cancun. From there you can take a taxi to the dock or walk out and wait for another bus to Puerto Juarez.
Once at Puerta Juarez, there are several express services currently serving the island. Detailed schedules and fares can be found here.