We did this excursion and absolutely LOVED it. You take the catamaran on about a 15 minute ride to a reef. You can still see the island and the ship but they look TINY. Anyway, the reef itself is absolutely GORGEOUS and the fish that live in it are amazing. We have snorkeled the lagoon twice but MUCH preferred he seahorse excursion to the lagoon (though the lagoon is a great place to learn for beginners). You snorkel for about an hour (which is pretty much the perfect amount of time to see everything) and then you head back. They have some snacks and drinks (water, sodas, punch etc) for the ride back. We had a really great time and considered it one of the highlights of our cruise!
Oh, we also did the afternoon one. If you are going anytime where the weather might have a chance of being slightly cool, I would go in the afternoon because the air temp will likely be warmer. Also if you get seasick you might want to take something for it before you go, as the catamaran is MUCH smaller than the ship and you definately feel the smaller waves (i don't get seasick but DH does). Also, have lunch at cookies as soon as it opens so you have time to digest!