This is not an idiom. An idiom is an expression whose meaning cannot be deduced from its elements. To go through fire and water for someone is easily understood to mean to make a special effort and to undergo difficulties for that person's sake.
It's not an idiom because you can figure out the meaning by context - you are willing to go through dangers or hardship for something or someone.
It refers to a pointless action. Another similar British idiom is " carrying coals to Newcastle". It means: You are doing something that will have no positive affect. you are wasting your time! Obviousely you cannot really carry water in a sieve, so when someone says that, they are basically saying that you are not acheiving anything.
A fish out of water is in an alien environment. If you say that someone is "a fish out of water," then you are saying they are out of their element. Stan the accountant was a fish out of water at the plumbing convention.
It means that something was in the water and has come out of it, or that something was transmitted through contact with the water.
It makes perfect sense, so it's not AN idiom.
This isn't an idiom. It is a proverb or saying because it means just what it seems to mean. Think about it. How could you use water if it's already gone down the river? If someone is always dwelling on the past, they're getting about as much done as a miller trying to use water that's already flowed past him. In other words, both of them are wasting time and energy on something that's just not going to happen.
You could draw a picture of someone standing in a swimming pool, with the water level being above their head.
That would have to be through water retention then, which can be caused by certain medications and illnesses. Not really something to aim for.
In trouble, usually yet to come. "You'll be in hot water when your parents find out" To be in hot water is to be in immediate, deep trouble, as if someone were cooking us in a pot on the stove. The person "in hot water" did a particular thing that is sure to make another person mad at him or her.
to wake up something or someone
It means that you are in"over your head," or in a situation you can't handle. It's similar to "in hot water." The image is of you over your head, in deep water, and having to swim for your life.
This is not an idiom I have ever heard. Perhaps you mean water cooler talk, which means chat that might occur between workers at the water cooler or anywhere else.