Taxis are usually parked next to the jardín (town square). It’s always a good idea to ask the price before you get in. Taxis are approximately $60 USD each way between the Manzanillo Airport and La Manzanilla and there are plenty of taxis available as you exit the airport. Manzanillo airport is tightly controlled, taxi rates are fixed from the airport and there is no bus service.
You can arrange for a taxi back to the airport a day or two in advance by talking with one of the drivers at the jardín. Taxis are also available to take you to Tenacatita for a day of snorkeling, and the drivers will usually just wait for you.