We did that shore excursion last week and LOVED it! As Michelle said.
There were about 200 of us on this and we met in the Walt Disney Theatre beforehand. Unfortunately
DCL still hasn't got the pre-shore excursion thing down pat and we were there for 20 minutes, most of which was spent repeating again and again for people to have their "letter size waiver" filled out and handed in. A table at the entrance would have been more efficient. People who have it, hand it in and go directly in, and those who don't, fill it in before going into the theatre. Just MHO.
Then Capt. Ron (or Capt Jack or whoever

) led us to the 2 catamarrans where we were split up. As we entered, we were given face masks and then flippers. The crew was very efficient and helpful. The Capt of our catamarran (Capt Freddy) gave us safety instructions and was very clear about it, said that soda, purified water (with purified ice) and soda were available now and after snorkelling, beer and margaritas would be served to the adults. The sky was a little overcast, but the snorkelling was good. Again, the crew were excellent, some came into the water to help those in need while others stayed on the boat to watch and they were very careful to do so. There seemed to be a lot of younger kids on our boat who had not snorkelled before and they were panicky. Some got back on as fast as they got in the water. We did this for 45 minutes. I would suggest going in when the Capt announces that there are 5 minutes left, as there is less traffic getting back on the boat.
A photographer took pictures of families and kids, that were processed while we were on the beach and for sale for $12 (a lot less than Rock 'n Roller Coaster's!). Their T-shirts were also for sale for $15, it was a good quality shirt and I bought one.
We went to a beach (don't know the name, sorry) for an hour and other catamarrans joined in around the area. We had to all go to the front of the boat to get in as close as we could and then walk in the water to the beach (only about 3-4 ft deep). The beer, soda and margaritas followed

and they sold cheeseburger meals for $7. These burgers were huge and could be split and there were a couple of salads as well. A beach volleyball game was in full swing, lots of floats were available, kayaks and some hammocks too.
Sailed/motored back in the beautiful Caribbean sun, danced to music, did a conga line where the crew would squirt soda into the kids' mouthes (a hole punched in the top) and they loved this. The crew brought around all of the drinks so you didn't have to get up as often.

The crew sand and danced with the guests and seemed to enjoy it as much as we did.
The Captain did mention (nicely) about showing your appreciation to the crew in the form of a tip and we were happy to do so. I thought this was great value for the money and would do it again in a heartbeat! I should mention that we went with my just turned 18 yr old DD and 13 yr old DS and they had a great time too. Patrick played volleyball with Dad and Meg and I sunned, floated and kayaked together.
Hope that helps!
Cathy