Begonia flowers are edible, but is the Waterfall Begonia flower edible?

I would expect anything within the genus to have comparable edibility. Flavor quality could vary all over though.

I assume you are talking about to humans because the ASPCA says begonias are toxic to dogs and cats:

http://www.aspca.org/pet-care/...

I had to Google Waterfall Begonia, the Waterfall name seems to be a registered series name for some Begonia boliviensis cultivars.

According to this very informative article "Begonia flowers are edible and in fact very rich in vitamin C. Their pleasant sweet/tart taste comes from the oxalic acid they contain, the same product that gives spinach and rhubarb their flavor. Since oxalic acid can be toxic if consumed in large quantities, it is best to consume begonia flowers in moderation."

https://laidbackgardener.wordp...