I did the Champagne Catamaran excursion in October. You take a 20 minute bus ride to where the boat is docked and then sail over to a small beach in St. John - the ride is about 40 minutes. Then you have an hour and a half to snorkel, lay on the beach, or stay on the catamaran. The snorkeling wasn't too thrilling, but I did see lots of fish and a sting ray (no turtles). I thoroughly enjoyed it. It was one of the most beautiful places I have ever seen, and the sand on the beach was as soft as powder. I wish the time on the beach was longer, but the time we had and the money we spent on the excursion was absolutely worth it.
I didn't find the ride back to St. Thomas to be an overly-party atmosphere. There were lots of families (I was there with my cousin) and it was actually kind of relaxing. Most people were pretty wiped from the snorkeling. The crew is really welcoming and they do bring out the rum punch and champagne and nibbles and encourage everyone to have a good time. Most people were sipping their drinks and enjoying the view. Nobody was wasted or anything. I think they knew that the excursion was from a disney ship so it would be a little more tame, rather than say a group of twenty-somethings off Carnival.
I would not hesitate to book this excursion again if I ever went back. The boat is clean, the crew is really friendly, and the gear is well maintained. The food and drink on the way back was a welcome sight - I forgot how much snorkeling can work up an appetite. Don't be fooled by "champagne" in the title though... It is not a luxury yacht
Whichever you choose, I hope you have a great time!