Yes, this is a wonderful site full of resources and great trip reports. We usually do a very short one, although I'm buying a new camera and plan to include photos next time.

Anyway, to answer your question--for peace-of-mind, make reservations--especially if there is a specific restaurant or activity you simply must do.
We always go at Christmas/New Years and have never had to make reservations anywhere. HOWEVER, we spend our entire vacation glued to a lounge chair at Club O beach and happy to eat pretty much anywhere. This prolly isn't a very helpful post--suffice it to say that there is no harm in making reservations, if you're concerned.

See you on the beach! Dec 14-24.