A few years ago my sister and I enjoyed a wonderful day at Blue Lagoon. We only did the beach option, not the dolphin encounter.
The beach had many hammocks (we snagged two right by the water!) and a swimming area. We didn't see any snorkeling, but that was okay with us as our hammocks were AMAZING! There were also games (volleyball and such) which were on the beach. Some were organized, some weren't.
The trip over on the boat was nice. There was a band playing and the views of the "bay" were great. The boat was a two deck boat with tables and chairs. Very spacious, but there wasn't a smoking/non-smoking section. The trip back was a bit crowded as there were two boats to choose from, a couple hours apart. These boats were more like ferries where you just pile in.
The food was okay. I don't remember us being terribly impressed.
Overall, we really enjoyed it. But now, we are hooked on snorkeling and head to Stuart Cove every time. (I am taking my mother there next week!) If you are really interested in snorkeling, I can highly recommend Stuart Cove.
Hope this helps and enjoy your cruise! (How could you not!)