Expressions about water are almost as common as water.            
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. When we are in “hot water”, we are in trouble.        A young boy can be in hot water with his mother if he comes into the house with dirty shoes.
      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.
Another common expression, “to hold water”, is about the strength or weakness of an idea or opinion that you may be arguing about. If it can hold water, it is strong and has no holes in it. If your argument can hold water, it is strong and does not have any holes.     
“Throwing cold water” also is an expression that deals with ideas or proposals.     For example, you want to buy a new car because the old one has some problems. But your wife “throws cold water” on the idea, because she says a new car costs too much.

A．It means disliking an idea.

B．You are in a difficult situation.

C．A person who breaks a law can be in hot water with the police.

D．If it does not hold water, then it is weak and not worth debating.

E. But many of the expressions using water have unpleasant meanings.
F. A man who has long been jobless will find it hard to make both ends meet.
G. “To keep your head above water” is a colorful expression that means staying out of debt.