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Many of the expressions using water have different kinds of meanings in English. "Throwing cold water on an idea" means "not to like an idea". For example, you want to buy a new car because the old one has some problems, but your parents throw cold water on the idea, because they think a new car costs too much.
"Water over the dam" is an expression about a past event. It's something that has finished and it can't be changed.The expression comes from the idea—Water has run over a dam and it can't be brought back again.When a friend is troubled by a mistake he has made, you might tell him to forget about it. You can say "It's water over the dam".
"Be in hot water" was used five hundred years ago to mean being in trouble. One story says it got the meaning from throwing hot water down on enemies attacking a town. That happens no longer, but we still get into hot water. When we are in hot water, we are in trouble. It can be any kind of trouble, serious or not 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 a house with dirty shoes.