On the contrary water does not work as “flush”. meaning that more you drink, the more waste products will be removed. Rather the removal of wastes remains the same as long as you are producing 500 ml of urine, the minimum required for waste excretion.
The kidneys do a dual job, they conserve water when body is in short supply, while they excrete water when you over drink it. And when you drink excessively you make the job of kidneys difficult, bcoz now kidneys have to excrete “water” while holding on to essential electrolytes. Excessive driniking over long periods can lead to inability of kidney to concentrate urine, which means that when you really need your kidneys to hold on to water, like in case of dehydration, bleeding, shock, they wopn’t be able to do so.
The myth of 8 glasses does not come from Quran or Hadith, its someone’s wild guess with no scientific basis.