"8 glasses a day" is completely unnecessary
Doctors have been saying it for decades, but that hasn't stopped pop culture from clinging ferociously to the idea that we all need to be drinking more water, all the time. And even that drinking more than the bare minimum amount of water can have healthful benefits.
First of all, that "8 glasses a day" number is complete fabricated nonsense. For most people, under most circumstances, you need about 2.5 liters of water a day - but you get most of that water from the food that you eat.
Fruits, vegetables, meat, dairy products - these all contain water. More of it than you might think. Coffee and tea count, too. (And no, it is not true that coffee will dehydrate you. Coffee is about 99.98% water and .02% other stuff.)
There are also no legitimate studies which show that drinking more water helps with weight loss or skin care. There are some studies which show this, it's true, but surprise! They were all funded by bottled water companies like Nestle Waters.
Your body is very good at letting you know if you are getting dehydrated. If you're thirsty, drink water. If you're not thirsty, then you're fine!