xhonzi said:

Hey! I said FERAL ghouls, FERAL! And I wasn't thinking about my mutant supermutant. I was mostly thinking about all of the ones I kill on sight. But the hitch is that it will also kill/make sick anyone suffering from any small amount of radiation sickness... I.e. basically everyone.

From the wiki:

In the DLC Broken Steel, if the player had chosen to contaminate the purifier with the Modified FEV, the medical facility of every city is filled with new patients dying from FEV poisoning. The new patients hold their stomachs and repeat dialogue along the lines of, "it burns on the inside." When the player speaks with the doctor at any given town, they state that the condition is not contagious. In Rivet City, Dr. Preston comments that they are burning the corpses just to be safe. In Megaton, Doc Church comments along the lines of, "it would be ironic if we were so used to the crap we had been drinking that pure water is killing us."

I also read that drinking the water will kill you... because you've obviously soaked up a lot or rads in your time.

Yeah, I just read all that myself. Really cool.

I know you said feral, but obviously it would kill the intelligent ghouls as well. Eden explains that it will kill any mutations, including just about everyone who wasn't born in a radiation free environment like the vaults. Which explains why it kills you if you drink it.