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 five hundred years ago to mean being in trouble.
One story says it got that meaning from the custom of extremely throwing 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, and 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 comes into 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.
You can be in deep water, for example, if you invest in stocks without knowing anything about the stock market.
