The doctor says "waterborne" when she just means "soluble".
The doctor talks about Aconite (Wolfsbane) poisoning and says that it is very hard to get hold of and difficult and time consuming to extract the poison from Aconite.
However to most people Aconite is just a pretty flower that can be found wild in the Alps and the Highlands and growing in people's gardens all over Europe. It's so common can be ordered online or bought from any flower nursery. And poisoning someone with Aconite is as easy as handing them a bouquet of flowers.