What will happen if you only drink water for a months?

You'll probably survive, but you won't have the energy to do just anything after a few days.

You'll lose a chunk of weight, but depending on your body composition, quite a lot of that would come from otherwise useful muscle tissue being used up.

You'll most likely get quite sick towards the end, as you run out of key nutrients and trace elements.

There's a big risk of damage to some internal organs, primarily kidneys and liver.

Getting back to a normal eating routine afterwards will be a challenge.

As a weight loss method it pretty much sucks. First it's the health risk involved, then, if you go back to your previous eating/exercse habits, you'll just pile the pounds right back on. But this time, probably even faster, as you'll have less muscle to use calories up with this time.