There are many ferries a day in the summer and usually one or two a day in the off-season. The ferries take about 9 hours from Santorini and stop at a couple other islands on the way, usually Paros, Naxos and Ios. There are a couple highspeed ferries that take half the time and cost twice as much. Making connections to other islands by boats can be tricky but usually there are daily boats to Paros, Naxos, Ios, Mykonos and Sifnos. There is an airport in Santorini with several flights a day to Mykonos and in the summer connections to Rhodes and Crete(maybe). There are even international charters from Europe in the summer. Ferry schedules are unreliable especially for boats between the islands. Unless you are a backpacker and don't mind having to improvise it is a good idea to have a travel agent book your ferries with your hotels. You will get a packet when you arrive in Greece with your hotel vouchers, ferry tickets and timetable and when you arrive in Santorini there will be transportation to your hotel. Otherwise you can get the schedules every day in the Kathemerini insert of the International Herald Tribune. The only problem is that they only have them for that day and by the time you pick up the paper half the ferries have already left. You can pick up a weekly schedule at the Greek National Tourism Office except they never have the whole week and they always tell you to call the ferry company to make sure it is correct. Don't be discouraged by the port of Santorini called Athenaios. Unlike most islands it is just there for coming and going and not hanging out in.

While in Santorini
There are frequent buses all over the islands and there are also taxis. In summer, buses leave Fira's central station every hour for Akrotiri and every 30 min for Oia, Monolithos, Kamari and Perissa.

Buses leave Fira, Kamari and Perissa for Athinios Port 1 ?hours before most ferry departures. Buses for fira leaves from Athinios Port after every ferry arrival, even late at night.

But a car or a motorbike is the best transportation way to see the island. The roads are pretty good.

But, be careful because every summer, lots of people has accidents.