We don't need to poison the population with contaminated water. We just need to keep them dumbed down and ignorant. And some other steps.
The first thing we need to do, is to build secret underwater bases inside bluewhales. Military funds along with private investors will give us the required resources. These bluewhale bases have to be sufficient to support 20% of the worlds population.
The selection for these bases will be done based on education, job and overall genetic predeposition.
To keep the remaining 80% ignorant and oblivious to the coming events and underground bases we need to arrange a series of disasters and terrible events. First we will create a virus, then take use of the media to hype up the virus as the worst pandemic since 1918, then use this to inject most of the worlds population with a vaccine containing various substances that cause neuro degeneration and chronic fatigue.
This will keep the population weak and ignorant.
Then we cause a world wide food crisis, thus starving hundreds of millions, and diverting focus away from our plans while we're at it.
To top off the food crisis we cause the total devaluation of the dollar and the american economy which will ripple through the world as a wave of famine and terror.
Then we venture inside our bluwhale hosts and wait for the planetary alignment to flood the earths surface and cause a pole shift to kill off the remaining parasites.