Acommon health myth is that people should drink at least eight glasses of water per day. while staying hydrated is important, why could this advice be inaccurate for different people?

How much you drink in a day depends on your size and weight. For example, I need only 33 oz. of water while a larger person who is much more active might need 50 oz.
In fact, it could be dangerous as being over hydrated could lead to sickness, nausea, and other problems associated with cells in hypotonic solutions.

Yes, this is very true. specifically causing damage in the  dentrite.