My kids LOVED Common Grounds and the counselors. Make sure your kids go to the welcome the first night to make friends. My daughter (17) and son (14) are not "best friends" at home but went to the welcome together. They started playing cards and other kids came over to join then - They wouldn't admit it but I think having each other made it easier for them to hang out until they met other friends.
When the age groups split, they were in different groups. This was fine because it gave them some time apart. My daughter had a buddy group right from the start, while my son was a little more of a loner - he attended and participated in his activities but sometimes just went with one other guy up to ESPN. But something I noticed, that REALLY AMAZED ME was that my daughters "gang" often invited my son along when they were just hanging out - taking picture out on the deck, etc. I'm not sure my daughter is that considerate of her brother or if it was the influence of the other kids.
I think they were very well supervised, but I have pretty good kids. There is nothing to make sure your kids stay with the group (i.e. no sign-in/sign-out) on a daily basis. So, a kid set on getting into trouble certainly could.
We had breakfast together daily and discussed the day's plans (they went on all excursions with us but we allowed them to go back to the ship earlier than we did). We then met again at dinner and discussed what was going to go on after dinner and what appropriate curfews should be (depending on what teen activities were planned). We also made it a point to meet the parents of most of the kids - met for a drink or in one of the clubs, etc.
The kids had such a good time that my daughter is trying to make plans with her best friend to go on a
Disney cruise when they graduate from high school next May rather than go to "senior week" (a local tradition at the shore).