I'll try this agin (my computer went crazy). I went on a cruise to the Bahamas and Puerto Rico with my girl scout troop the summer before my senior year of high school. I think I was 17. I don't know if I would let my own daughter go or not based on the way some of our troop members behaved. We were totally unsupervised. I, like your daughter, was a straight A, straight laced student, and didn't do anything particularly bad, but there were only two of us that "behaved" out of the whole troop. While you are supposed to be 18 to drink on the ship, most of the girls ordered drinks without being carded (with the exception of two of us). Our ship's waiters took us to a bar in Puerto Rico, and some of the girls went across to the other side of St. Thomas to a secluded beach with some of the staff members from the ship. Me and a couple of other girls stayed in the "touristy" part of the island (but probably would have gone had we been in the right place at the right time). As far as I know nothing inappropriate happened. THe scary thing to me is that something could have happened. They got lucky that the guys they went with were nice. One of my friend's got sick and puked in one of the public bathrooms on the ship. Our parents let us go on that cruise thinking that we were going to be on a nice clean supervised girl scout trip, and as far as I'm concerned, our leader let them down.
That being said, we had a really good time and I'll never forget it. Two of us did behave ourselves and still had a good time. Maybe your daughter is like me and won't participate in things she shouldn't. (Granted, even though I "behaved" for the most part, I still went to the bar with them even though I didn't drink). Maybe her leader has more control over the troop, and maybe her friends have more self control than mine did. Don't assume that your daughter won't behave, because she might.
That's my input. I would make sure that you really trust the leader before letting her go. My daughter is only 11 months, so I have awhile before I have to make these decisions.