When we went in January, our dd(10) was signed up to snorkel. She is NOT a sporty type kid, and easily gives up things she is uncomfortable about, but she does like to swim. I was absolutely amazed at how much she enjoyed snorkeling. And it was so special to swim along side her and look at things. She and her dad went in the morning, and I joined her in the afternoon. Even though I thought things look great later in the day, she said the morning was beautiful, and you could see colors better. We've since bought snorkel gear for all 3 of us, so we can do it together.
Also, have you considered doing the banana boats on CC.....They were just about the most fun thing we did as a family......can't wait to do it again!!!