We're gonna go out on a limb (or out on a pier, perhaps) and say that water is hands-down the biggest symbol in the Tao Te Ching, where it's used to represent the great flow of the Tao itself. Over and over again, the TTC uses water to teach us lessons on how to be more like the Tao.
Here's the first mention we find:
The highest goodness resembles water
Water greatly benefits myriad things without contention
It stays in places that people dislike
Therefore it is similar to the Tao (8.1-4)
"Highest goodness"? Whoa, that's pretty high praise coming from the TTC, which is usually talking about how we should think of everything equally. But notice here what's so great about water: it gives to everything equally, without raising a ruckus about it. Like the Tao, it makes no judgments and gives to all. The awesomeness of water doesn't stop here, though. Check out this quote:
Nothing in the world is softer or weaker than water
Yet nothing is better at overcoming the hard and strong (78.1-2)
So water is awesome because its weakness makes it strong? It may sound like mumbo jumbo at first, but it's totally true. Eventually even the highest mountain can be worn down by water. Water's very softness is what allows it to creep into the cracks and wear down anything. If you've ever had a pipe burst in your house, then you know the damage that water can do.
So, just like the Tao, water has the power to flow into everything, but it does so with ease and grace. (Sometimes it also does so as a giant tsunami, but that doesn't quite fit this analogy.)
Okay, one more water quote because we just can't help ourselves:
The great Tao is like a flood
It can flow to the left or to the right
The myriad things depend on it for life, but it never stops
It achieves its work, but does not take credit
It clothes and feeds myriad things, but does not rule over them (34.1-5)
Here the translator uses a simile to show how the TTC compares the Tao to a flood. Like water, the Tao is flexible; it can flow in any direction. Also like the Tao, water is humble. Every single life form on Earth laps up water like nobody's business, but you don't hear water asking for recognition. When was the last time you heard the water in a flowerpot ask for an award or something? Yeah, it doesn't happen. Water gives to everything selflessly… just like the Tao.
Really, all this water imagery and symbolism is one big analogy. The TTC uses something familiar to us—water—to explain something that's a lot harder to understand: the Tao. And what's the purpose of making the Tao more relatable? So that we can try to be more like it. So that we can see what's cool about being humble, flexible, weak, and non-judgmental.
Man, the next time we take a shower is going to be a whole new experience.