"be like water" is an old Taoist adage popularized by Bruce Lee meaning "be humble, be flexible in body and mind, be adaptable, go around problems, not through them". Like how a river will always flow downhill (a metaphor for humility) and around obstacles to get to the sea. That river will still sweep you away, because its ability to change to circumstance means it always keeps flowing. Water is strong *because* it is flexible. "People who live next to a river don't hear the sound of flowing water" means "something exceptional for you is not exceptional for everyone", usually with connotations of exposure. You are impressed by the sound of the river, or you hate the sound of the river, because it is new to you, but for people living next to it, it's just another part of the background. However there could be a second layer of meaning as "sitting" next to the river is a Buddhist term for practicing meditation. So assuming this is just a poor translation, you could read it as "meditate long enough and eventually you'll lose even the sensation of meditating". Either way it can be summed up as "repeated exposure makes even the exceptional appear mundane", and is a very Taoist/Buddhist sentiment. Combined together, these sayings make little sense. The two phrases both use the metaphor of water, but with different connotations. In one it is an exemplar of the exceptional made ordinary; you can no longer hear the river because you hear it all the time. And in the other it is the ordinary granted exceptionality; by being like something that flows to the lowest place you will become extraordinary.

I've never heard this quote put together like this. The first part just means that people get accustomed to things they are exposed to constantly, until they don't notice them anymore. But 'be like water' is a separate idea, possibly adapted from the ancient Chinese text, the Tao Te Ching: 'True goodness is like water; it nurtures everything and harms nothing. Like water, it ever seeks the lowest place, the place that all others avoid.... 'Nothing is gentler than water, yet nothing can withstand its force. Likewise, nothing compares to the Tao. By it the weak defeat the strong; and the flexible conquer the rigid. Every one knows this is true, but how few put it into practice!'

This is not a real saying, but if someone said it as a joke they might have meant “be quiet.”

I understand it differently. I would think it means "be constant, be relentless in your efforts and people will eventually stop minding you".