How big is your group and when are you sailing?
We were a group of 14 on the sold-out Wonder over spring break. One family had reserved a regular cabana but then realized sharing it (as in...you get wristbands for the capacity of the cabana) was not going to work, she went to cancel it when we got on the ship. Of course, she didn't realize there was a 3-day prior cancellation policy. The girl helping her disappears (as all pixie dust seems to happen behind closed doors) and came back and asked if we would prefer the family/double cabana. We split it four ways and had a great time. But we called over and over and were told there was no availability. I think they were holding it until embarkation day.
The service is as you will read in reviews. They are very nice. They are slow to come when you ring and slow to bring drinks. The walk to the bathroom and lunch wasn't bad--use the path in front of the cabanas, not the beach. We had trouble finding someone to drive us down there and walked too far. But we were off the ship very early.
As far as the lagoon, it was pretty shallow until later when the tide came in. I did not swim in it. The kids found a conch in its shell. They lost it after lunch, found a new one and the old one. They had a great time near the water rather than in.
Good luck!