Every visitor to Istanbul wants to—and should—take a day cruise on the Bosphorus, the 32 km (20-mile)-long strait which joins the Sea of Marmara to the south with the Black Seato the north of Istanbul.
Your cruise can take as little as 90 minutes, or the entire day; may just be transport across the water, or may include sightseeing and lunch. There are many varieties:
Short & Frequent
TurYol operates frequent 90-minute cruises that take you from Eminönü up the Bosphorus to the Fatih Bridge, then turn around and return to Eminönü. The trip is transport-only, with no stops nor commentary, but it allows you to see much of the city in a short time for a low price. More...
Şehir Hatları, the big traditional Istanbul ferryboats, run a short tour cruise (Kısa Boğaz Turu) from April through October. It's a 2-hour round-trip departing Eminönü and Üsküdar daily in mid-afternoon, going north of the Fatih Bridge making no stops enroute, then turning around and returning to Eminönü and Üsküdar. Fare is TL10.
Dentur Avraya operates 2-1/2-hour cruises from Kabataş, making several stops at docks along the way.
Full Day Cruise
These are the big traditional Istanbul passenger ferryboats beloved by Istanbullus. Take your choice of:
—The Long Tour (Uzun Boğaz Turu), a 6-hour round-trip cruise departing daily at 10:35 am from Eminönü and 15 minutes later from Beşiktaş, calling at Kanlıca, Sarıyerand Rumeli Kavağı before arriving at Anadolu Kavağı, the northernmost dock on the Asian shore. Enjoy lunchat one of the many seafood restaurants near the ferry dock during the 3-hour layover, then cruise back to the city, arriving at Eminönü in late afternoon.
—The Moonlight Cruise (Mehtap Gezisi), a two-hour round-trip leisure cruise, departs Saturday evenings in the warmer months. Fare is TL20.
The number of voyages depends on the time of year, with fewer in winter, when the few voyages will probably be in the middle of the day; and summer, when there may be hourly cruises throughout the day.
Kabataş is probably the most convenient dock to board the cruise as it is at the end of the Kabataş-Bağcılar tramline. Boats depart Kabataş at 10:30 am and 14:45 (2:45 pm) daily, with an additional cruise added many afternoons at 16:45 (4:45 pm).
Many other boats are ready to take you on a Bosphorus cruise. Two smaller companies, EMTUR and Boğaziçi YolTur, launch their cruises (TL10) from next to the Şehir Hatları temporary Bosphorus Cruise Dock just east of the Galata Bridge at the Eminönü (southern end) of the bridge (map).
There are yet more boats. Walk west from the Eminönüend of Galata Bridge, past the Golden Horn ferry dock(Eminönü Haliç İskelesi) and toward the big T. C. Istanbul Ticaret Üniversitesi building and you'll see dozens of boats moored along the shore advertisingBosphorus cruises and charter trips. I don't know these boats and can't vouch for their reliability.