Drinking a lot of water can make you feel bloated and fat but the fatness or bloating from the water is only temporary and will go away once the water is passed through your body.
Drinking water is a good way to actually lose weight because the water helps your body feel full so you won't want to eat as much.
When you don't eat as much food but yet still feel full then you'll lose weight because you won't be consuming too many calories.
Fasting and only drinking water for a few days is a good way to help burn off fat and lose weight.
You should also exercise as well to burn off fat.
But if you're trying to gain weight you should eat more calories which helps you gain weight.