How well do LEGO assemblies hold water? 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.