Actually, the fast jetboats make the run from Capri to Naples in about 50 min. The slower ferries do take about 80 - 90 min. But, unless you are taking a car there is no reason to use the slower ferries unless you want to save a few dollars. There is currently a fast boat at 6:50 am, but that is today and it could change tomorrow. It is possible that bad weather could delay or cancel a boat not to mention the always chance of a labor strike. Since your flight is in the morning you might want to consider spending the night near the airport or perhaps in Sorrento.