Expressions about water are almost as common as water itself. But many of the expressions using water have unpleasant meanings. The expression "to be in hot water" is one of them. It is a very old expression. "Hot water" was used 500 years ago to mean being in trouble. One story says it got that meaning from the custom of throwing extremely hot water down on enemies attacking a castle. That no longer happens. But we still get in "hot water".When we are in "hot water", we are in trouble. It can be any kind of trouble--serious or not so serious. A person who breaks a law can be in hot water with the police. A young boy can be in hot water with his mother. if he walks in the house with dirty shoes. Being in "deep water" is almost the same as being in hot water. When you are in deep water, you are in a difficult position. Imagine a person who cannot swim being thrown in water over his head. You are in deep water when you are facing a problem that you do not have the ability to solve. The problem is too deep. You can be in deep water, for example, if you invest in stocks without knowing anything about the stock market. "To keep your head above water" is a colorful expression that means staying out of debt. A company seeks to keep its head above water during economic hard times. A man who loses his job tries to keep his head above water until he finds a new job. "Water over the dam" is another expression about a past event. It is something that is finished. It cannot be changed. The expression comes from the idea that water that has flowed over a dam cannot be brought back again. What can you infer from the passage?
"To be in hot water" can only be used to mention serious trouble. "Water over the dam" is an expression about a future event. Water could be used in the war in the past. All of the expressions using water have unpleasant meanings.