That's right, if you don't book an excursion you can't get off the ship. I did this cruise last November and stayed on the ship.
The Gatun Lake Yatch Club is not like a Yatch Club as we know it and you have to book the excursion to even go over there. From pictures that I have seen on the video and from comments from friends who took the excursion, it is just a big tent with craft tables set up with goods to sell - people dancing and singing (saw a couple of topless ladies in the video) - that's the entertainment.
Some peoplesaid they went swimming in the lake. Our friends just went over to get off the ship, but they said it wasn't worth what the shorex office is charging for the excursion.
Just remember the weather in the Canal is extremely hot, so make sure you have lots of sun tan lotion and wear a hat.
I did this excursion while onboard the Rotterdam this past January. The Yacht Club isn't much to speak of but it is well worth the $25 excursion price. It's not advertised in the literature but included in your ticket is a bus trip to the Gatun Locks viewing area. Be prepared to climb a good number of steps to reach the viewing platform but it is well worth it. You're about 200 feet from the ships that pass in front of you through the locks.
This piece of information is conveniently left out of the literature because they want you to pay for the much more expensive excursions to visit the locks. It is indeed hot and humid but the viewing platform is completely shaded.