Capri

The direct ferry from Positano to Capri and back - depending on the time of year you are traveling.

There is a company that organizes daily boat excursions from Positano down at the beach/harbor. Years ago I took it from there and I liked it. Ask the local restaurants on the shore, they'll direct you to the right place. I'm not sure there is a regular ferry from Positano. You may need to go to Sorrento for that.

Like ekc said, there are direct ferries back and forth between Positano & Capri during the high season, which is roughly mid-May to mid-October. Just go down to the ticket booths on the Positano beachfront & you'll find them. No need to return via Sorrento unless you want to.

Apparently I was able to find something from Positano to Capri. It is probably only for summer though. Departs from Positano at 10:00 (70 min). the Return departs Capri at 16:25 back to Positano then Salerno. The cost is Euro 14.50 (I presume one way). These are for summer 2012, next year I don't know.
Of course there might be more choices from Sorrento. http://www.salernotoday.it/guida/trasporti/trasporto-navale/orario-traghetti-salerno-amalfi-positano-capri-1-aprile-2012.html

Found more boat excursions online. This one would leave at 9 am for 35 euros (probably per person round trip). So apparently there are several companies. However I think they are all for the summer months, not for winter. For more go to Google and enter these key words:
TRAGHETTI DA POSITANO A CAPRI http://www.coopsantandrea.com/it/servizidilinea/servizi_b.asp?id=40

Chris, I took the trip from Positano to Capri when I was last in that area, and it's very easy. The boat is not a "Ferry" in the usual sense (no cars are carried), but rather a passenger boat. You'll need to go down to the beach and then stay on the right side to go out on the concrete jetty. You'll pass by ticket booths on the way. When you buy your ticket, be sure to note the time of the last boat returning from Capri. You can somewhat see the arrangement by having a look at These Webcams (during daylight hours of course - it's dark at the moment). I can't remember the exact length of the trip, but as I recall it was about an hour. I believe the cost is about €18 each way. You'll arrive at Marina Grande, and you'll then have to buy a ticket on the Funicular to get up to the main part of town. Trying to get on the Funicular back to Marina Grande in the late afternoon can be a bit of a challenge, as it will be packed. If you find that to be the case, take one of the open-top Taxi's. It' a pleasant ride and you'll avoid the crowds. Happy travels!