When ancient Chinese first invented their pictograph for "water," they simply drew a little picture that depicts flowing water, like this:
This pictograph is found on some oracle bones dating back to 3,000 years ago. This word did not change much over time, and you can easily see the the resemblance between what the character used to be and what it is today:
Whereas mountain stands for commitment and principledness in traditional Chinese culture, water symbolizes flexibility and adaptiveness. It is for this reason that Confucius said that "The intelligent man enjoys water, and the humane man appreciates mountain." Daolist philosophers had great appreciation for water. Lao Zi, the Old Master said: "The highest virtue is like that of water."
A person with too much flexibility and adaptiveness may be one who lacks character and loyalty. There is the Chinese set phrase "shuǐ xìng yáng huā” - “the disposition of water and popular catkins." This phrase has been used to refer to women with loose morals (the saying is somewhat sexist since there are evidently quite some unprincipled men in this world too).
水平 [shuǐ píng] literally means "water level" or "level like water." In addition to the meaning of "level" or "aptitude," the term also implies justice or fairness because water is level wherever it stays. When a Chinese protests against some unequal treatment, he may say "yī wǎn shuǐ duān píng,“ "hold level your bowl of water." Your bowl may be tilted, but water in it remains level; if you tilt your bowl too much, water will spill.
A relatively new phrase that contains the character for water is 水军 [shuǐ jūn], literally ”water army." This is an idiom in today's internet culture, referring to organized trolls who post online to shape opinion or create publicity. They're so called because they pump a lot of "water" into public forums.
Another idiomatic term, associated with commerce, is 水货 [shuǐ huò], "water goods." This refers to merchandise smuggled into China. Such goods mostly come over water, hence the expression.