The reason you shouldn't drink water on an empty stomach is because it can lead to nauseousness and stomach upset as your stomach tries to digest the water when it expects food.
When drinking water on an empty stomach your stomach wants to digest the water like it would food so it leads to nauseousness.
It's good to eat something before drinking water or anything else to allow your stomach to digest that first and then drink the water or whatever else you're drinking.
The reason you feel nauseous after drinking water on an empty stomach is because of the lack of anything in your stomach for the enzymes in your stomach to digest.
To stop feeling nauseous when drinking water you should eat something such as a bowl of cereal, apple, banana, oatmeal etc.
Anything in your stomach before drinking water is better than nothing.
As a result of your stomach being empty your stomach will perceive the sudden water intake as food and release gastric enzymes to digest it.
Without food for the stomach to break down, the incoming water stimulates the rapid production of stomach acids, causing irritation and nausea symptoms instead.
When you drink a significant amount of water, particularly on an empty stomach, the stomach distention stimulates more acid production, which can result in heartburn.