Minerals in the rocks/mud/silt on the bottom of the waterway and minerals dissolved in and /or being carried by the water.

Surroundings (trees, skies, sunlight, sharks)

Not sharks @rebbel.

It also depends on if the water is warm or cold, fast or slow, and what types of algae, moss, what-have-you grows in that environment.

I’d imagine whatever is in, below or above the water would affect how it looks.

Some rivers in Anchorage were a milky turquoise color from glacial silt.