Weather permitting, there are at least four boats daily from Piraeus, including car ferries and HighSpeeds and SeaJets, some of which take cars. Check ferry schedules with a travel agency, or with the Piraeus Port Authority (tel. 210/451-1311, -1440, or -1441; phone not always answered) or Sifnos Port Authority (tel. 22840/33-617). Ferries travel on ever-changing schedules to other islands, including Serifos, Kimolos Milos, Tinos, Paros, and Kithnos. Boats are notoriously late and/or early; your travel or ticket agent will give you an estimate of times involved in above journeys. www.openseas.gr is a useful resource for boat schedules. If you have time to kill waiting for your boat, have a seat by the Poseidon Café, where an ouzo and mezedes were a reasonable 2€ in 2011.
Tip: If you find yourself waiting (and waiting) for your boat in Kamares, the Emba K'Evas (Come and Go) Café has tables and chairs by the water, good coffee, and ouzo and simple mezedes for a very reasonable 2€.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.