It is not an idiom, it is fact. A hose can be used for a water level with hundreds of feet between the two points. As long as two bodies of water are connected somehow, they will be at the same level. As an idiom, it would mean a person tends to seek out people they feel are equal or have the same point of view.
Stay calm
A level of craziness comparable to that of a bedbug.
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.
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.
First, when you see the phrase AS ___ AS ___ it is A Simile instead of an idiom. It is easy to figure out the meaning of a simile. Water flows quickly, so the simile means that whatever it is, it's as fast as water flowing.
The idiom "level-headed" refers to someone who is calm, rational, and able to make sound decisions in difficult situations. It describes a person who remains composed and clear-minded, even under pressure.
The idiom of biggest wheels is often seen shortened to big wheel. A big wheel is a person who has power in an organization, including lower-level bosses, managers, and CEOs.
As a duck takes to water means that you learnt very quickly or immediately and enjoyed the experience
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
It is not an idiom, it means your nose is itching.
blow a gasket, blow a fuse : pushed past some tolerance level into an angry fit.