I agree with the first reply. Since our port was Santa Apolonia, it would NOT have been walkable. So make sure they don't change the port on you. I imagine it will be printed on your paperwork when it comes.
Edit: To put that another way, a cruise ticket is not a binding promise to deliver you to a particular city on any particular day. One time in Central America, our ship-tour bus came back late, and the boat had moved to a different dock to free up the one we got off at. Think about that when you save money on a non-ship day tour ...