We've lived on Ischia, one of the other islands in the Bay of Naples, for almost a decade - and similarly use hydrofoils and ferries to go ashore... many of them the same vessels that also serve Capri.

For an idea of the difference, try:

http://www.pbase.com/isolaverde/caremar

Whilst there are often hydrofoil sailings that get cancelled because of the weather during winter, the ferry ships are often able to get through - but, that said, mid-October is still autumn, and cancellations of either then are much rarer!

The Capri authorities aren't keen on late shipping movements, and I'm fairly sure there won't be many later arrivals in that season than at present:

From Naples:
http://www.capri.net/en/ferry-schedule?path_id=11

From Sorrento:
http://www.capri.net/en/ferry-schedule?path_id=14

In previous summers, the Caremar line has been allowed to run a 21.10 ferry from Naples, however I never noticed quite when it's withdrawn again!

http://www.regione.campania.it/portal/media-type/html/user/anon/page/CTTD_DettaglioNews.psml?itemId=20&ibName=NotiziaArea3&theVectString=0%2C-1

But, for 2011, it's shown here as only running till 30 Sept..

http://www.capritourism.com/imgg/download/orari_marittimi_09092011.pdf

Whilst I wrote elsewhere of the plans for 2012...

http://www.tripadvisor.co.uk/ShowTopic-g187785-i421-k5722596-Naples_to_Capri-Naples_Province_of_Naples_Campania.html

... subsequent events saw a very odd pattern of autumn sailings (nothing that late at all in October) - see their timetables for September 2012 onwards, here:

http://www.caremar.it/caremar/pages_it/orariloaded.asp?Porto1=NAPO&Porto2=CAPR&anno=2012&mese=9

So, whilst you may be lucky, it might be wisest to also look into the alternatives?

Peter