Do I need to drink 6-8 glasses of water a day for my health?

Short answer; no.

It's actually not known how the idea started that drinking 6-8 glasses of water every day is absolutely essential for good health, but it certainly has gained a lot of traction over the years.

Generally, when your body is becoming depleted in fluid volume an involuntary thirst reflex engages which makes you feel thirsty. This results in you getting something to drink so you no longer feel thirsty. In other words, natural mechanisms keep you from becoming unknowingly dehydrated so that it isn't necessary to drink fluids constantly to prevent dehydration.

Mind you, this assumes general circumstances. Certainly, if you are trying to lose weight it helps to feel full on no-calorie water instead of food.

Also, very elderly people can lose some of the thirst reflex and not realize that they are mildly low on fluids.

Also, this does not apply in special circumstances where you may be losing more fluids than usual or where access to fluid is restricted, such as strenuous work outdoors or diarrheal illness (or desert combat operations for those of you in the military). Here, you want to be mindful to replenish fluids often enough to urinate every 2 hours and for your urine to look clear or light yellow. Also, be sure to use sport drinks or eat food to replace lost sodium.

If you are concerned about dehydration, the first sign of this is a decrease in urine production and/or dark colored urine. Improvement in this is a good sign of adequate rehydration.