It will be hot and humid. Personally, I would hate it, but some people (obviously) find humid heat less oppressive and unpleasant than others. Generally December would be less prone to truly cyclonic weather than later in the summer (although there have been cyclcones in December).

The box jellyfish "season" is under way in December in the tropical waters off Queensland (south as far as Gladstone or so), and I certainly think swimming at the beach would not be a sensible option (for me). I am not sure how much the jellyfish are found in the waters of the Reef itself, however.