I think that the crowds in December, on both the island and the main beach, might be a little higher than average since it's a popular month. But even at full capacity, it never seems crowded. I think that being naked on your balcony isn't frowned upon as long as it's done discreetly. Like you said, most are enclosed. But for those on the lower floors, you might want to throw a towel over the balcony rail, so those walking under you don't get an eyeful. This is what we do if we happen to get a room right above the lobby.
I would put a towel on the rail. The way they were remade is the front edge of the board on the ocean side is lower than the other side room side. If you are near the front of the balcony you can be seen from below if they look up.
If you are in the South wing, no one can see, no towel required.
My husband and I visited CTI in September and our room was on the 4th floor main building-we spent most of our time on the balcony in the nude and never noticed any gaukers so I'm assuming you couldn't see. Most of the balonies are enclosed enough that you can see very little. Remember, it's your vacation make it your own - if that includes being nude on your balcony do so just remember to conscientious of the other vacationers. If nudity is your thing don't forget to visit the island I can assure you no one out there cares is you are nude - LOL!