I have been to Nassau a couple of times and it is not my favorite port. But, when you can only go on a 3 day, that is about the only place to go. It was quite a few years ago- I don't remember that much about what we did- but I don't think we went snorkeling.
No- my kids will not stay on the ship at CC. I was talking about Nassau. If we go snorkeling in Nassau, I will leave them on the ship. Maybe my daughter will come with us. Depending on how the time works out, maybe we will take them to the pirate museum, or the beach.
My kids are not too shy. I don't think they will have any problems going in the kids clubs. At least I hope not! We will be with them all week at WDW before we go, so it will be nice to have a break. Hopefully the counselors will help your daughter meet a few kids?