How to get Sifnos: One or two boats per day from Athens (Pireaus harbor).  Fast boats (catamarans or “flying dolphins”) take less than three hours and are comparable to airline travel.  Slow boats take about five hours and have restaurants, bars, and breazy open air decks.  The one-way fare to Sifnos is between 4,000 to 9,000 drachmas ($11-25 USD), depending on the boat and class of ticket.  

Here is the link to the Greek Ferry system (in both English and German!)

http://www.greekferries.gr/

This link will take you through a two-step process to pull the ferry schedules for the day you are traveling. Please be aware that due to high winds, occasionally the high-speed ferries do not run.