I've not done it, but I have seen it. The boat is a monster sized speed boat with huge (loud) engines. They take you on a ride that includes at least one snorkel stop and a tour around the island. Appears to be fun (I personally do a sailboat /snorkel tour that I find quite nice, I don't need the speed and noise when I'm in the Caribbean).
We did the "wild thing" a few years ago and it was great fun. Not sure of the exact snorkeling spot they will take you, but we ended up in St. John/Trunk Bay, but I understand they don't always go there. If you want a fun time, go ahead and give that excursion a try!
I did the Wild Thing and we did not get to St John's either. It was fun.........we went over to Mullet bay and the snorkling was not as good as it could have been. If that is what you are looking for, I would opt for one of the other choices\