Ios is famous for its summer nightlife in Chora and the long sandy sweep of Mylopotas, but it is also a lovely, rugged island once the season quietens down. Either way, the trip back to Athens is a long one, and it pays to choose your boat with care.
Three companies make the crossing. Blue Star Ferries is the steady, year-round option, sailing five days a week in every month; Fast Ferries also runs all year, on fewer days; and Seajets adds a much quicker high-speed boat from May to October. Because the conventional ships call at several islands on their way north, the gap between the fastest and slowest sailings is large, from under five hours to over nine.
Many conventional sailings from Ios leave in the evening and reach Piraeus the next day, and the timetable ranges from very early departures, as early as 04:30 according to Ferryhopper, to late ones at 22:00. That can work well: a cabin on an overnight ship saves a hotel night and lands you in Athens ready for the day. Just check the arrival time before you plan anything in Athens.
Piraeus is Athens' main port, about 15 km from the city centre and 45 km from the airport. If you are flying out, the fast boat or an overnight ship arriving early gives you the most room. In summer, book early, as cabins and car spaces on the conventional ships are limited.