I know I'm in the minority, because I read the good reviews, but we did it in November and didn't like it much. The boat is very crowded, and we thought the snorkeling stop was really bad. They said it was a reef, but all I saw was sand. And sand usually equals not much to see. We tried to swim to a better area, but they are quite strict in keeping everyone in a contained area. If you're snorkeling for the first time, you may think it's okay. And if you're a beginner, I noticed they really gave no instructions on how to snorkel. The ride to the beach from the snorkel stop seemed long. The water toys at the beach were really difficult to climb on. I don't blame them for that, but it wasn't real enjoyable.
Again, I know most people enjoy this excursion, but this is just our experience. In fact, some of the people sitting around us seemed to enjoy it. If you're an experienced snorkeler or hate crowds, I believe you'll be disappointed.