Istanbul Shore Excursions are the best ways to explore this city. And a night cruise is one of the best.. Istanbul is like a shining jewel at night. It becomes a totally different city under the lights.
If it's summer (I hope it is!) you're really in for a treat, the bridges will be lit (sometimes very boringly over-lit though..), the weather will be wonderful, not very hot, not chilly, just perfect, and you will have your booze on board..
If it is fall or spring, it can be chilly after the sunset, so be cautious and have a jacket handy..
If it is winter time, well, there are better things to do in the city, don't freeze yourself!
Just tell the captain where you'd like to see, he will adjust the cruise dependingf on your wishes..
you can choose to let the captain decide if you don't want to prepare a route.
They are very experienced and will cover all places that worth seeing..