The Silversea cruise ship will either dock at one of two ports in Istanbul. Depending on your cruise itinerary, the ship will be scheduled to dock the Karakoy or Sali Pazari Passenger Terminal. Once you depart the ship at either location, there are usually a number of taxis available to drive you to any shore excursion destination you wish to visit. Most attractions are no more than 30 minutes away by taxi, and it's advised to negotiate your taxi fare with the driver before accepting offer of their service.
For more info, see our feature Istanbul Port FAQ