I was on the Eastern in Aug '02. We did not sign up the "tour", but did something similar and much cheaper.
As soon as you get off the ship, you will find a few independent tour operators. You can strike a deal for a private tour on your own for a much cheaper price. They ask for much less than the excursion price, and I was able to work out a deal for a private (and comfortable) car for just my family. Even after a very generous tip, we saved quite a bit, and we were able to work out a drop-of along 7 Mile Beach at the end; the ride back is a very inexpensive taxi ride. Another benefit was that we were able to stay longer at places we liked and move more quickly at places we weren't interested in. Another benefit is that you'll get an island's perspective from a native who isn't being scripted (or censored).
You will not get a guided tour of the turtle farm, but there is a self-guided tour w/ signs explaining each step of the process. There was a guided tour going through when we were there, and I caught a bit of it. I didn't think that we were missing much, because the guide wasn't saying anything that wasn't on the signs! And, the kids mostly wanted to play with the turtles anyway!
Some people are going to be nervous getting off ship at a strange port where they've never been and going off with a stranger. They appreciate the security and predictability that come with a Disney-arranged tour, and pre-arranged excursions are ideal for them.
My family has always opted for less structure when we travel. Sometimes we will miss something that we would have caught on the guided tour, but our most memorable travel experiences have been the ones were we went off the beaten path.
