The Sydney Harbour Cruise will be the Captain Cook Tour by Captain Cook Cruises. The cruise will begin at 4 pm promptly. Boarding finishes at 3:50 pm, so you should aim to arrive no later than 3:45 pm. The cruise will finish at 6pm. The boat will depart from and return to King Street Wharf 1 in Darling Harbour.
The Google map below is centered on the location. The closest train station is Wynyard Station. Click the map to launch a larger version.