Try this site
Just returned from three days at Positano The ferrys run every day and there are at least three operators. You book on the main beach 24 hrs before. It was 50 euros a head for a whole day with The Blue Grotto extra as it is not always open. The boats leave between 9-10am and return between 5-6pm. Some boats dock at the jetty while one tour group ferry you to a larger boat from the beach.
They go everyday regardless of weather and I saw one boat returning on a very rough day. I could not believe they were going to be able to get into the beach to disembark over 40 people in really rough conditions but they did without any mishap.