We are going on a 16-day full Panama Canal crossing in a couple of weeks. In answer to the OP's question, I think it is determined by whether or not you are doing the "full panama" or "partial panama".
We're taking a Celebrity cruise, and this is what I just learned (a fellow cruiser asked if we would be allowed off at the start of the Panama crossing for an excursion)...
For your ship and itinerary -- excursions will be offered while in Panama Canal.
The ship will be cruising through some of the locks until it arrives at Gatun Lake. Here the ship will stop so that the guests with shore excursions can be picked up and transferred to the Gatun Marina where they will commence with the excursions.
When the shore excursions guests debark in Gatun Lake the ship will cruise to Cristobal Pier where it will dock and wait for the guests with the excursions.
While the ship is docked Cristobal Pier guests who did not purchase a shore excursions may debark the ship and wonder on their own.
Unfortunately at this time the tour operator has not provided us with the shore excursion information for the Panama Canal, there for we have not uploaded those onto our system as of yet.
This information should be uploaded no later than the 1st week of September
We are truly sorry and would like to apologize for any inconvenience this may have caused
Should you have any questions please feel free to contact us."
Shore Excursions and Explorations! Dept