There are ferries from Puerto Juarez which is North of Cancun which run all day and ferries from Cancun's hotel zone which run less frequently and cost about twice as much. Puerto Juarez is a 25-50min taxi ride from the airport, depending on traffic.
At the time of me writing this, the cost from Puerto Juarez is 160 Pesos (US$8) per adult (130 Pesos = US$6.50 per child) for a one-way trip and 300 Pesos (US$15.50) for adults, 250 pesos (US$13) per child round trip.
Please see this website for up-to-date information.