We've done it twice. Once in 2005 and once in 2006. We thought it was great each time. I would probably rate it a 7 out of 10 or so when it comes to snorkeling boat trips I've taken.
Goods:
A lot more fish than CC and more variety
Clearer than CC
There's coral (none at CC)
Views of ship are great coming and going
Bads
Takes away from beach time
Not for beginners
Limited snorkeling time (less than an hour)
I am an experienced snorkeler and I got the shivers when I saw a shark and a very large barracuda. My DS11 got scared because the water between the boat and reef was maybe 20' deep.
I remember that we took a Catamaran out to the reef. Ride was smooth and crew gave a speech about snorkeling and the rules. Took 30 minutes or so. I don't remember any food...but maybe we had water. I don't remember needing any food after our CC meal.
Boat moored next to small reef (looks like an underwater island of coral in a sea of sand). We could snorkel all around the reef area...maybe the size of a basketball court. One of the crew paddled around on a surf board to keep us all nearby and to help snorkelers if needed.
After about an hour they called us in and we motored back to CC.
We'd do it again. I wouldn't recommended for kids under 8 or so unless they have snorkeled some and are comfortable with their mask.
In 2006 when we saw the shark, I told the crew and he didn't really believe me. I'm assuming it was just a sand shark and it wasn't very big.