I am sure he means the air temp is 19.
If you have 3'+ depth and a deicer to keep a hole open you will be fine. As for the waterfall, a pond have to be pretty deep for significant temperature stratification to occur. WHat you need to be really careful about it that ice and snow doesn;t start to build up around the sides of the waterfall and divert the flow out of the pond. (This happens to people alot, even after years of having no problems.)