From Athens to Mykonos by ferry - From Athens to Santorini by ferry
Buy the ferry ticket online, in advance. It's easy, fast and safe. And you do not run the risk of discovering that the boat is crowded on the day of departure for another island (important in the high season, July and August). A great place to buy them online is Let's Ferry
On all ferry sites the prices are the same, but in this, there is no extra surprise when you complete the purchase.
There are categories that you must choose (for example the Silver category), but keep in mind that most times it is not worth it to pay more. After the boat leaves you sit where you want.
There are two types of boat: one express, which takes around two hours between Athens-Mykonos or Mykonos-Santorini, and another slower, which takes 4 hours each way. You should consider taking the quicker one, cause four hours of doing nothing is a long time, even inside a super comfortable bar in the middle of the Mediterranean.
In Athens, there are two ferry ports to the islands: Rafina and Piraeus (these are the names you look for on the website when you buy your ticket). Piraeus (say:Piraeus ) is the closest to the center, about half an hour drive.
Be careful though: According to the law the tickets have to be printed out only by certified travel agencies or the ferry companies themselves so even if you book it online you have to visit a travel agent to take the print outs anyways (this can even happen in the Piraeus port though just outside of the ferry before boarding – there are kiosks in port that do that if you give them your booking number or printout)!
The best website to book your ferry from Athens to Mykonos or from Athens to Santorini is : Let's Ferry