How much water we need to drink every day?

You know those ads that remind us that even a small reduction in levels of hydration tremendous impact on performance , so you need to keep hydrated with whatever brand of super isotonic drink that they sell ?
They seem quite scientific. Are they for real? A man in a white coat , an athlete with electrodes attached and so on . And it is not hard sell because drinking feels right - you're hot and sweating , no doubt where the fluid must be beneficial .