Stopped in Santa Marta in November on an NCL cruise. Why cruise ships stop there is beyond me. The town and area around it is very dirty and not even a shopping area. Nearly all of the excursions were hours away and up in the hills, which my brother says were lovely but long bus rides. We should of stayed on the ship but it was docked next to a coal export harbor facility so you couldn't sit out on the balcony. Abysmal.
I didn't get a ferry ticket but I did exchange one excursion for another with no problem. I think they want it done a day or 2 before but they can be pretty easy going. There is a separate 800 number that takes you to an excursion agent. You can get that by talking to an NCL booking agent.