1, drinking the kind of water that is carbonated drinks.
If you drink boiled water, it won't make people fat scientifically. Because there are no nutrients in boiled water that can make people fat. Therefore, in theory, it will not "gain weight."
However, if the "water" you drink is not water in the traditional sense, but various commercially available beverages, that is another matter. Most commercial drinks contain sugar and the content is not low. The average sugar content is as high as per 100 ml 10 g. A bottle of 500ml sweet drink, with a sugar content of about 50g, provides 240kcal of energy for the human body, which is equivalent to the energy of a bowl of ordinary white rice.
2, it may be that the spleen and stomach are not good
What we eat or drink must be transported by the spleen and stomach before being transferred to other organs. If your spleen and stomach are weak and the transport function is poor, then what you drink or eat can't be transported, and it will accumulate in the body. If it goes on for a long time, it will naturally accumulate into fat and gain weight.