How is the weather in Dec? Will all the hotels have heating? In winter will there b a river cruise?

It will be a little bit cold at that time, especially when it rains. But the scenery is still OK. There is air-conditioner in hotels, so do not worry about it.
On Yulong River, there is the cruise. On Li-river, the crafts are still in operation, but the there may not have cruise at that time.
In Yangshuo, maybe there is celebration at that time.