I went last year over thanksgiving break (ohmygosh its been a year!!!!!!) and it was for my graduation. I had the best time.
We had two fancy nights, which I loved dressing up for. Then shorts/tanks/sundresses/bathing suits/sandles. A dress would be useful on land, but it gets REALLY windy on the ship, so shorts/tanks are better for those days. It should be warm, it was cold when we left miami, but by the next day it was in the high eighties and nineties. The ship can get VERY cold though, so def bring a hoody if you are like me and get cold easily.
I hung out in the teen club a lot but almost everybody was 16-17 and some 18(they were very lenient with that....) and so it was an older crowd. I basically just went there at night where we danced and such, since we couldn't get into the older club. What ship are you going on????
Cozumel=amazing. I don't know how your parents are, but check out carlos & charlies or Senior Frog. Fun stuff. & for Grand Caymen just the shops are pretty entertaining. Almost everybody goes and sees the stingrays which are nice as well. You will have so much fun!!!!!