"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.
When a friend is troubled by a mistake she has made, you might tell her to forget about it. You say it is water over the dam.
Another common expression, "to hold water," is about the strength or weakness of an idea or opinion that you may be arguing about.
It probably comes from a way of testing the condition of a container.
If it can hold water, it is strong and has no holes in it. If your agrument can hold water, it is strong and does not have any holes.
If it does not hold water, then it is weak and not worth debating.
"Throwing cold water" also is an expression that deals with ideas or proposals. It means to not like an idea.
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.
