We just hopped on the regular 1.5 hr cruise ferry at Eminonu, the one where the usher keeps yelling "Bogaz Bogaz Bogaz". They depart often, and the cost is 10 lira each, paid to the usher. There's tea and juice served on board. Many bring food and sesame bagels.
I'd recommend to bring some source of information on what's on the sides of the Bosphorus, so you can read about things as you pass them.
One and a half hours seemed just right. The ferry stays close to the European coast and goes non-stop to Rumeli Fortress and the suspension bridge, then makes a turn and heads back staying close to the Asian coast. Departure at 5.30 pm proved to be a perfect timing: the sun was low and cast golden light on the European side, then we saw the sunset hues on the way back, and landed amid the night illumination.
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