It might be summer in Australia but it doesn't mean hot! I have been here three years. In the same week, you will have 33 degrees then the next day it plunges to 12.
Every summer, I keep putting duvet away, duvet back, away,back! and the water is freeeeeeezing even in January. If you look at a map, we are quite close to the South Pole! I am in Sydney.
I have been maybe 15 times to the Canaries for christmas. You have good years and bad years, but even when warm it is never hot. In the evenings you need a jumper AND a jacket .
And the is heated pool and heated pool.
The warmest water I have swam in was in Bali. Christmas time. Like a hot tub. A pure bliss. I would spend my day in the water. We had some showers but the weather was nice and hot. Thailand is nice as well.
Asia is nice and cheaper than the Caribic, unless you want those massive resorts, then you have more choice in the Caribic.
Egypt used to be the best spot for Christmas for short haul. Both Sharm and Hurgada are hot and water is warm. But it is not safe anymore. Don't even think about taking the risk, because even if nothing happens, your stay will be crap. Tourism has plunged, staff is reduced to minimum, maintenance ditto and they sell last minute holidays to Russia for $100 a week, and prepare food accordingly. The cheapest and worst.
Asia is cheap, nice and warm. Go for it.