He is to me as water to a fish

Liu Bei said that to Zhang Fei and Guan Yu when the two derided him for putting his trust into Zhuge Liang before even testing him.

This quote, besides the relationship between lord and advisor, has many meanings.

For one, a son, daughter, father, husbanad, cousin, etc., cannot be part of a family if there is no family. That will result in a title without meaning. Families are water in which the member fishes exist in.

For another, the relations between friend or friends. Each cannot make do without the other. This will prove later in the story after Liu Bei dies. Each friend can take the place of the two options in question with no fixed role.