we stayed on paros last september and there were plenty of ferries running down to santorini from pireaus calling at mykonos/paros/naxos then santorini so should be no problem, try looking at greek news travel pages (gntp) site they run most ferry time tables . kev

Mykonos-Santorini is in part a 'cross route', so direct ferries are not always frequent, a guesstimate of 1/day in September, Hellenic Seaways. Alternatively take a ferry Mykonos-Paros 1/day and then change to Paros-Santorini about 3x/day. Bluestar ferries and others.

There will be no definative timetable before August, it's the way it is with greek ferries.

http://www.gtp.gr/RoutesForm.asp
http://www.bluestarferries.com/ENGLISH/defaultmain.htm
http://www.hellenicseaways.gr/index.asp?a_id=117
http://egov.yen.gr/en/yen.chtm?prnbr=27622
http://www.ekathimerini.com/4dcgi/_w_articles_info_11_00/00/0000_43