OH! Dong Dong , you have it so wrong wrong, December is not a good month the learn to surf in Bali it is the rainy season every cesspool of a stream enters the sea from the top of Mount Agung all the way down.
here is what Big H writes about this December
viewtopic.php?f=6&t=25188.
The Sanur side and Nusa Dua can have better waves then but since so much water as oceanic currents move through the Lombok Strait , it is dangerous.
One advantage less crowded so you won't be as much of a hazard in the water
There are still local guides available to get you to some where safe wave wise but have your health check and ways of keeping healthy. Consider too wet season means mosquito borne diseases , malaria Dengue, encephalitis to name a few. Talk to your doctor toensure what protections you need!
You might not any choice of the when but if you can choose the dry season April to October with the mid months being peak surf season, either end is shoulder so surf less crowds etc!
Good luck

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷