Assuming that you will be entering the Canal from the Atlantic, Princess is referring to the shore excursions that depart from Gatun Lake after the ship has gone through the locks on the Atlantic side. And yes, you must be on a ship-sponsored tour. This is true for all mainstream cruise lines.
I’ve done four full transits of the Canal, but never a partial, so I can’t advise you on the specific excursions, but the Canal of course is not far from the Equator, and it will typically be a hot and humid day.