We booked the Royal Caribbean Catamaran Double Site Snorkel & Beach.
The Good: The Reef is very healthy. We were able to see a lot of sea life and coral, including sea turtles, a spotted ray, squid, as well as your usually fish life. Clarity of the water was good. We also had a long time in the water, about 50 minutes. The crew were very helpful and friendly. We enjoyed the Beach at Playa Mia. My husband went out on the Hobie Cat and My son wandered around the water park. I hit the beach.
The Bad: This isn't a true sailing Catamaran. They have disabled the sail and they motored the whole way out. You should note that it takes about 45 minutes to reach the snorkeling site. Also, there really is only one snorkeling site. They swim about 20 yards over to the "second" site-- so not really two snorkel stops. There really isn't enough time at the beach, once you eat, you have only about an hour. There was no way to do everything there.
The OK: Again, you are supposed to follow the guy in the water, who has a flag. However, we pretty much did what we wanted since we are strong swimmers and snorkelers. It is important to note that the water was a little rougher here, but the guide follows the natural current, so it's not too tiring for those who are not strong swimmers. The lunch provided was just ok. Not bad.
I wish this excursion was longer. Overall I give it a B+