It has nothing to do with how pools work...

It's about how visible wavelengths work...

Edit:

To actually explain it, red wavelengths are shit at going thru water

Send light to the bottom of a pool and it reflects back as just blue off white.

Send it to the bottom of even a shallow pool that's dark blue, and you get back black. Which warms the water and helps algae. With the green algae, you get even blacker water.