Here is a description I wrote in response to another post. Jewel of the Seas was the ship that the OP was referring to, other ships do partial transits as well. Most partial transits originate from Florida, but Princess also does one out of California.
Partial Transit: I think it can be a very satisfying cruise. Be warned that once you have done a partial you will probably want to go back and do a full transit and that is normal! The Jewel sails into the Atlantic entrance of the Canal and locks up into Gatun Lake through the Gatun Locks in the morning. The ship will arrive at the anchorage in Gatun Lake around 9 to 10. This is where the tendering takes place, passengers who have booked ship's tours are tendered off to meet their tours. Note, that only passengers that are on ship's tours are allowed off here and the ship's tenders do not complete the transit. The ship will then lock back down Gatun Locks and proceed to the Cristobal piers where they have a very nice shopping area inside the pier. The pier is also where those who are on tours will rejoin the ship.
If you want to get more of the Canal there is one tour called something like "Canal by Ferry". This tour takes you to the Pacific side after you have tendered off the ship in Gatun Lake. You will sail briefly in the Pacific, (the approach channel) as well as Miraflores, Pedro Miguel Locks and Gaillard Cut. This tour will take you through about 80% of the Canal. The only portion of the Canal you won't see is the 20 mile stretch of Gatun Lake from Gamboa to Gatun Locks. You will also be able to see some of the country side on the bus ride to meet your ferry.
Full Transit: The whole enchilada, Atlantic to Pacific or vice versa.
The good thing about a patial is that if you get off the ship and take a tour you are not really missing a lot since you return through the same locks, leaving the same way you came in. I would not schedule a tour on a complete transit if I had not been through the Canal before. I think they are both unique cruises, even if you have done one, taking the other is rewarding as well.
"If the phone doesn't ring, it's me!"
"Is it ignorance or is it apathy? ... I don't know and I don't care!"
S.S. Badger R/T from Manitowoc June 02