Ferrying around the Greek Islands.

From mid-May through mid-September, you can generally count on daily boat connections between islands. Outside these times, service is dramatically cut back, especially after November 1st. Strong winds start blowing in mid-September, often suspending boat services. Try not to have too tight a schedule planned.
Always plan at least one night in Athens before your flight back home or to your next destination. There are good hotels near the airport so you do not have to travel into Athens itself.

The Greek islands are divided into several groups. The Cyclades are the most well-known. Boat connections are usually good, so long as you can stay in the same group of islands. Hydrofoils are twice as fast and twice as expensive. Flying between islands is another option, though you'll usually need to fly via Athens there are not many direct flights between islands. As a general rule, flights cost double to triple the ferry fare.