Passenger ferryboats from Istanbul sail regularly to the Princes Islands (Adalar). (Car ferries do not go, as private cars are not allowed on the islands.)
Departures are from the Eminönü, Kadıköy and Bostancı ferry docks (map), by three different companies: Şehir Hatları, the traditional ferries; TurYol smaller ferries; and Dentur Avrasya ferries.
Note that there are no shuttle boats among the islands, only the scheduled ferries. To travel from one island to another (for example, from Büyükada to Heybeliada), you must check the ferry schedules to find voyages that will both take to the other island, and take you away from it so you don't end up spending the night (unless you want to).
Şehir Hatları Ferries
Şehir Hatları vessels, the big traditional Istanbul ferryboats, make the 100-minute voyage from Istanbul's Eminönü ferry docks to Kadıköy, Kınalıada, Burgazada, Heybeliada and Büyükada (map) at least 10 times daily (more in summer) for a fare of TL5.20 single/one-way with an Istanbulkart, TL7 without.
There are also at least four ferries daily between Bostancı, on the Asian Sea of Marmara shore, and the islands, for the same fares.
On many summer days, and on all warm-weather weekends, the Şehir Hatları ferries fill quickly, even though they can hold upwards of 1500 passengers.
It's best to take one of the earliest ferries, and to be at the ferry dock at least 15 minutes before departure (30 minutes is even better) in order to get in line to board. When boarding begins, passengers near the front of the line can choose the best seats: interior, exterior, upper deck or lower deck, on the sunny side or the shady side, in the bow or the stern.
On busy days, it's best not to plan on taking the last or even second-last ferry of the day to return to Istanbul, as these boats fill to capacity (or over-capacity) early in their voyages. You may not be able to board, and if you are able, you may have to stand during the entire voyage in crowded conditions.
For example, if you plan to leave Heybeliada for Eminönü on a late afternoon or early evening ferry on Saturday or Sunday, you may find that the ferry is filled to capacity when it arrives at Heybeli, and you won't be allowed to board.
An alternative route to and from Büyükada is to take the IDO seabus from Bostancı (map), a suburb on the Asian coast of the Sea of Marmara. You can reach Bostancı by seabus from Yenikapı, or by Metro.
TurYol boats depart the TurYol Karaköy ferry docks at the northern end of Galata Bridge, on the west side by the fish market, in the warm months, especially on weekends. Schedules meet demand. More...
Dentur Avrasya ferries make the voyage between Istanbul's Beşiktaş docks and the islands at least six times daily in both directions in winter, even more frequently in summer.
—by Tom Brosnahan