It’s no surprise that Istanbul is well served by ferries. With the exception of the leisure-oriented Bosphorus cruises, ferries are most useful for crossing the Bosphorus (rather than going up and down it) and for getting to the Princes’ Islands. The main docks on the European side are at Eminönü and Karaköy (on either side of the Galata Bridge) and at Kabataş, while Üsküdar and Kadıköy are the most important docks on the Asian side. Traditional large ferryboats operated by Şehir Hatları (www.sehirhatlari.com.tr), as well as smaller, somewhat faster-moving ferries run by two private companies (Turyol and Dentur Avrasya), crisscross the Bosphorus day and night, and cost about the same as land-based public transport. The Princes’ Islands are served both by Şehir Hatları and by İDO, which operates "sea bus" catamarans from Kabataş that are faster, sleeker, and completely enclosed (www.ido.com.tr).
Taking a ferry is also one of the best ways to get in and out of Istanbul. "Fast ferries," some of which carry cars, leave from Yenikapı, which is south of Aksaray and a short taxi ride from Sultanahmet, to various ports on the southern side of the Sea of Marmara. The most useful routes are the ferries to Yalova, Bursa, and Bandırma, for travelers heading to İznik, Bursa, and Çanakkale, respectively.