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The hydrofoil (ferry) is alongside where you dock in Naples. Unless you have a long day stop in port you may find it a rush job to do both in a day. Personally Sorrento would be my choice if you have to decide on one or other, about 15 euro return. Here's a site you can copy & paste http://www.capri.com/en/orari-traghetti.
How about a plan B? You could go Naples to Sorrento by hydrofoil and back on a train (see - http://www.massalubrense.it/circum.htm). The trains are regular and timely. The website http://www.sorrento-online.com/english/orari.htm also gives good info. From Naples station to port takes 10 mins and costs about €10, and if you do it this way round you wont get hassled by the taxi drivers at the port, and they can be a real pain in the bum!!!!
Have a good holiday .
If you decide to do Capri by the Hydrofoil, you can only purchase a one way ticket. As soon as you arrive in Capri go to the ticket office and purchase your return ticket for later in the day. The little funicular train that takes you up to the top of the island will be crammed and have long queues so try to time your arrival before or after the main ship excursions. You could then go by hydrofoil onto Sorrento and back from Sorrento to Naples by Taxi/Road. Capri has lovely views but very expensive designer shops. Sorrento also has spectacular views and lovely cobbled streets full of interesting (affordable shopping) and bars/cafes. My first choice is with Wilbas - Sorrento. Especially if you can find a good taxi driver - we had one called Mario who sang to us all the way! and even gave us a CD to take back home with us. I think I may still have his card if you would like his contact details let me know.
How long is the taxi ride and what was the cost.
We didn't go to either but went to Ischia instead-a pretty island and much less commercialised and busy than Capri. Easy to get there, after coming off the ship at Naples just walked a few minutes to the ferry ticket office at the port where the ferries to Ischia and Capri run from regularly.
Yes it is possible but ensure that you allow sufficient time to get back to Naples to sail in the evening. Would suggest you get off the cruise ship asap and get your ticket from the ticket offices adjacent the quay.Be aware you will have to pay to get a bus/taxi up the hill to sorrento as it is a fair climb by foot. also you need time to get up on the rail lift to Anacapri then a taxi/bus up to the top. It may be better looking at the excursion if you want to enjoy the day without worrying and fighting against time.