This bowl judgment will almost certainly follow directly on the heals of the second bowl judgment and will up the ante, so to speak, by removing the last known good sources of clean water available to wicked men and women on the planet. Again, this should not, at least in theory, affect believers at the time as God will almost invariably give them clean water to drink despite this judgment, but those who have taken the mark will absolutely without question be forced to drink this tainted water and, like the seas that were turned to blood, this polluted water will also putrefy and cause the entire planet to stink of death, rot and disease.
Speaking of which, all this rotting blood and dead fish, animals, and other creatures, will in turn cause even more disease to spread across the planet like wildfire. So not only will those with the mark be suffering from all the prior plagues and bowl judgments, they will also have to face the onset of many terrible, deadly and veracious diseases that will weaken and decimate their ranks. And the thing is, there's worse than that yet to come. While this is bad, what comes next is even worse.
(*Do you have a better description, or did you find an error? Please let me know.