To be honest even at end March/begining of April we have found the pools very cold. The sea itself is actually a bit warmer, but if you intend to do a lot of snorkelling in December you might need one of those shortie wet suits if you feel the cold.

Hope you get a reply from someone who has actually been in December, but going from what I have experienced in April it wouldn't be for me if swimming in pools was high on my agenda.

The only way to be sure is to pick one of the more expensive hotels that state it definately has one heated pool open all the year round. I think the Hilton Sharks bay, the Holiday Village and the Coral Sea resort have heated pools, but it depends on if you want these type of family style AI hotels.

We're off again on the 11th December.
We have been twice before. Once on the 5th and once the following week, just before Xmas.

Both times the sea was warm, however the wind does blow and there were quite a few red flag days.
I have been told the sea actually cools in Feb/March but it has been nice and warm for us which is why we are going back.

The pools are another matter. The first trip was to the Conrad and the pools there were logged as being 20 degrees however I think it was measured in Fahrenheit! No one used the pools.
The second trip was to the LTI Grand Azure chosen because of a heated pool. The heated pool was like a bath, but the other pools were again too cold to use.

There was a lot of activity at the LTI ready for Xmas / New Year but according to the reviews the actual events were somewaht disappointing to the people who paid all that extra just for the privilege of going a week later.

Check the reviews of whatever hotel you are thinking off and see if anyone has Xmas experience of it.

We're back to the Conrad this year as they now show up as having a heated pool and they also have simple easy access to the Coral which is world leading.