What a lovely setting! Kizkulesi (aka The Maiden Tower) is such a wonderful place for a romantic dinner and then drinks at the top of this four story lighthouse. The tower is located just off the shore of Uskudar. You can get a ferry to Uskudar and then walk to the loading dock. When you are done you can hop on their boat taxi to either Uskudar or Kabatas.
Dinner is a little pricey but worth the romantic setting and the wonderful view of the city at night. Also, a lot of local couple go there so you get to see another side of the city.