Where to get off the Havaist airport bus to catch a ferry to Buyukada (Princes' Islands) in Istanbul?

Clarification on bus 15C (which stops at Kuzguncuk en route to the Üsküdar ferry): it's the same bus whether boarding from Kuzguncuk or Üsküdar, but Üsküdar is the terminus stop, so you have a better chance of getting a seat there.

Bus 139 to Büyükada departs from the Şemsipaşa terminus in the Üsküdar area, located on the waterfront within walking distance of Marmaray exits and ferry piers. Payment is by İstanbulkart at a premium fare (over 100 lira). Note that the bus runs only about every 2 hours.

From the city centre, take tram T1 to Sirkeci station, then board the Marmaray for one stop to Üsküdar — this is faster than taking a ferry across the Bosphorus.

Marmaray fares are distance-based: a registered İstanbulkart is charged for the actual journey, while an unregistered card is charged the maximum fare to the end of the line. A trip from the terminal stations (Halkalı or Gebze) costs around 75–76 ₺.

To stop at Kuzguncuk en route, take bus 15C toward Üsküdar ferry terminal and get off at Kuzguncuk-Şehit Samet Uslu stop. Cross the road and walk along İcadiye Cd. toward the colorful houses.

From the airport, buses 15U, 14M, or 15F head toward Üsküdar ferry terminal. Note that the walk from some stops to the pier is uphill and about 3 km — it's better to stay on the bus until the terminal itself.

If you prefer not to take a ferry to Üsküdar first, the Marmaray commuter rail line also stops at Üsküdar station, from where you can easily connect to ferries heading to Büyükada.