For the major cruise lines the itineraries are set. There are legal reasons you may not be able to even board in your city, even though it is a stop on the itinerary. (Passenger Services Act)
The cruise will end in Port H. If and I mean "if" you could board in your port (C) you would need to buy passage on the next cruise in order to get back to your city. That is if the next voyage included your city in the itinerary.
Cruises are different than bus routes.