I was under the impression (but I should test it more) that non-transparent water rendered faster than transparent. And I know that's a fact for turbulant water. I can't remember now what actual differences were, but I probably didn't choose doing it this way just for the fun of it. For water like this you wouldn't notice a difference between just a color variation plus reflection, or a calculated 'depth'. I only had to give the shallowest areas a color close to the dry ground, and deeper water a darker color.
And for this animation we wanted the absolute minimum in render time. For foliage and avoiding flicker it still needed to be really high, but transparent water might have (not sure) upped that even more. Well that's the story
By the way, did you notice the train (upper left) and the cyclist (near the end, coming along on left path)? And the water lilies; procedural.