Theory:

Idioms are a group of words that are established by usage and are unique to the language. It usually has a different meaning than what it reads. It is a set expression or a phrase, and it does not hold the same meaning when read separately. Idioms do not have literal meanings.
Example:
1. A snake in the grass.
In general terms, it has its usual meaning. But when used as an idiom, the meaning refers to a disloyal person who pretends to be good.
 
2. To come off with flying colours.

It means to emerge out with brilliant success, but not the literal meaning of coming out with a colourful display!
Next, let us look at some more idioms with meanings:
  
Idiom
Meaning
Example
Boil down toAmount toThe facts of the case boiled down to a mere fraudulent act.
Get into hot waterGet into difficultyJames got into hot water because of his laziness.
Shed crocodile tearsPretend to cryHe shed crocodile tears when his boss  scolded him.
Peas in a podStay together alwaysSheela and her sisters are like peas in a pod
Green with envyExtremely jealousHe was green with envy when his neighbour secured the first rank.
Idioms related to water:
No.
Idiom
Meaning
1
Water under the bridgeSomething that happened in the past and cannot be changed but is no longer a source of concern
2
As dull as dishwaterToo boring
3
Dead in the waterUnable to function in the usual way
4
Get into hot waterGet into difficulty
5
In deep watersIn trouble
6
Like a fish out of waterIn an uncomfortable, out of place situation
7
Mouth-watering Delicious
8
Keep (one’s) head above waterTo barely keep up one's position to stay ahead
9
Still waters run deep A quiet manner that conceals a passionate nature
10
Pour cold water on somethingFeel or say something negative about a situation
11
To throw the baby out with the bath waterTo accidentally discard something useful, while disposing waste
12
Tread waterFail to make progress.