THis is a modern day myth, there is no proven benefit of stuffing your stomach with water.
I am not denying that person would stay healthy if he/she dont drink 8 glasses of water daily...i drink about 3 and i do consider my self healthy...however if you look at this biologically the you ll realise that water intake work as flushing system. The more you intake the more rubbish you flush out...and this is absolutly not a myth in western countries...you can visit any doctor and they will always tell you to drink more fluid...at least in UK its a common practice that they never prescribe you antibiotic and tell you to drink more fluid...and as I gave references from Quran and Hadith which fluid could be better then water.