If you are riding your car and you come to saty in the island for at least 2 or 3 days you may consider crossing in the ferry that brings the goodies into the island. Don’t do it if you are coming for a short time since making the line and the ride itself is time cosuming of your precious vacation.
If you arrive to Cancun airport and you are concerned on budget I would advise to take the Riviera Bus “ADO”departing from the airport and bringing you right into the main plaza in Playa del Carmen where you’ll take the ferry. Be careful with their schedules though, and leave some extra time since they have been departing before the hour, and for some reason, they seem to be arriving in Playa, just on time to miss the next boat! (ferries change schedules every month, check the update); there are 2 companies taking the shift: http://www.mexicowaterjets.com/eng/index.php and http://granpuerto.com.mx/EN/index.html
Otherwise, there are also the regular taxis, (quite expensive) and the shuttle (that will depart when it is full and drop many persons before you)
Finally, there is also the plane that has some flights of 20 minutes from Cancun to Cozumel: http://www.mayair.com.mx/ From the airport in Cozumel it is a five minute ride to Amaranto.
If you arrive by the ferry you are only at 10 minutes from Amaranto,walking, but you may take a taxi if you are loaded. Bring of course the adress and phone numbers.