Human remains can be somewhat toxic (particularly to humans).
Human remains are a breeding ground for disease that specializes in breaking down humans. Now imagine that diseased juice getting into the local water system.
It's a myth that human remains are toxic. A human body is only as toxic as the living human that it once was. Which is to say, avoid getting blood and feces on your mucous membranes and you're not at any elevated risk.
With regard to drinking water, here's what the CDC advises with regard to whether people should be concerned with a natural abundance of un-preserved corpses in the wake of a natural disaster:
"Bacteria and viruses from human remains in flood water are a minor part of the overall contamination that can include uncontrolled sewerage, a variety of soil and water organisms, and household and industrial chemicals. There are no additional practices or precautions for flood water related to human remains, beyond what is normally required for safe food and drinking water, standard hygiene and first aid."
In general their broader advice is that you don't need to worry about any sort of disease outbreak happening if there are un-preserved corpses lying around (a position with which the WHO concurs).
We should be more concerned with formaldehyde leaching out of preserved corpses than from any chemical byproduct of human decomposition.
Human remains are a breeding ground for disease that specializes in breaking down humans. Now imagine that diseased juice getting into the local water system.