The Bosphorus Strait is a politicaly and naturally must see in Istanbul. Lots of countries wanted to get the controll of this strait in the history. When you look closer It connects Europe to Asia. But when you look at from a distance it is just a little detail of this huse world.

Crusing the Bosphorus gives you the opportunity to watch the city from the sea.
There are options like smaller, private tours or by ferry, there are many different options for a Bosphorus Cruise depending on price and duration.

The public Sehir Hatlari ferry is the most comfortable one starts from Eminonu. It travels the length of the Bosphorus, stopping at Besiktas, Kanlica, Yeniköy, Sariyer, Rumeli Kavagi and Anadolu Kavagi. Passengers can get off at any stop, however, most wait until the end, for the village of Anadolu Kavagi.

Another option is the modern and comfortable TurYol cruise, which is operated by collective of private boat owners in competition to the traditional public ferries. TurYol operates its Bosphorous cruise fromEminonu- look for the white ticket booth marked ‘TurYol- Bogaz Turu’ – and returns passengers to the Galata Bridge.
Private tours are another option. When you go to Eminönü, the sellers will try to sell their tours to you. They will ask for 3 hour tours for around 25- 30 TL per person. But if you bargain you can reduce it to 15 TL. They travel to Rumeli Hisari for lunch, and then turn back towardsEminonu.

There are also tour agencies that organize bosphorus cruises, with dinner and entertainment. Some of them are:
http://www.lufer.com.tr/Sayfa/Oku/yemekli-bogaz-turu-(aksam)
http://www.bogaztekneturu.com/bogaz_turu.htm
http://www.istanbultekneturlari.com/tekne-ve-yatlar-kiralik-tekneler