I will be flying to Rome, then taking the rapido train to Naples on May 21st. My plan is to then taxi to the harbor and catch a hydrofoil or fast boat to Positano, where I will be staying, rather than take a bus to Sorrento, and then take another bus to Positano. I see that NLG or Consorzio lmp serve the port of Naples, but can't seem to tell how often they go to Positano in a day. Do you think I'll have a problem with this in May? Also, any problems with the same transportation from Positano to Capri, or will I have a lot of choices?
Ferries from Naples to Positano are run by Metro del Mare. Here's a link to their website: http://www.metrodelmare.net/ If my very limited understanding of Italian is correct, they haven't yet published their May schedule. Unless I'm misreading, the companies you mentioned don't serve Positano. Here are their websites: http://www.consorziolmp.it/ http://www.navlib.it/index.asp
Last year the Metro del Mare only ran in the height of the season (financial decision). Am in Sorrento at the moment and no-one expects it to be any better this year but who knows! Alicost run services from Amalfi and Positano to Capri but have yet to post their schedules, check again after Easter which is the traditional time for the tourist services to start up. http://www.alicost.it/