There's no strong evidence for a universal 8-glass rule. Food provides water too, and coffee and tea count. Thirst works well for most healthy adults.
Evidence & sources
There is no single daily water target for everyone; most healthy people meet their needs by drinking when thirsty, and food supplies about 20% of water intake. Evidence: established. Known: Conclusion of the National Academies' water review. Unknown: Needs rise with heat, exercise, pregnancy and illness. Source: National Academies (Institute of Medicine), Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005).