Savings of 50 to 75%? That is a bit optimistic in my experience. You can get tours for less than the cruise lines offer, but not always. For instance, on Carnival it is/was cheaper to buy a ticket for the Playa Sol Beach Break in Cozumel from the ship than it was to take a taxi to this beach and buy your own admission.
And more importantly, as Karen noted you put yourself in peril for missing the ship when taking private tours.