Water is commonly represented with blue pieces, but what if I tried adding real water to a setup?
For example, I create a simple blue base with a rectangle making a sort of pool. If I fill that rectangle, would the water be held inside the pool?
Example 2, make a river using taller pieces as the bank and run water over it.
I already saw that LEGO bricks can float so maybe adding a nifty crocodile into a moat or something.
In my experience, this doesn't work very well. The gaps between LEGO bricks are too wide to hold water. The water will be contained for a short amount of time, but it will eventually leak out through the cracks.
Here's an experiment showing how your first example works. Notice the water leaking out the crack in the bottom right:
The leak continues to grow:
I haven't personally tested this with running water, but I can only imagine that this would increase the rate that water will leak out.
If you're interested in building some sort of running water feature, you may want to take a look at this answer that suggests waterproofing the bricks using liquid latex after assembly.
I did something like this once, I cut down a butter tub and built the lego around it, with plates hiding the top edge. It worked very well, but the stale tea that I used for the swamp stained my bricks.
I can only think that a brick-built wall with a plastic carrier bag secured by a plate layer at the very top would be your best bet, as none of the bricks would be modified, and the shape can be completely random.
And as you can see, both crocodiles and sharks float nicely.
