I think the point is that water is only technically blue; in small quantities like those simulated, its appearance is mostly the effect of refraction and scatte

I think the point is that water is only technically blue; in small quantities like those simulated, its appearance is mostly the effect of refraction and scattering and all that fun stuff, which is a whole new can of worms. So they add some blue (or green) partly to make it more obvious what's going on, but partly to cover up for the optics not being quite right. You can see it especially in the last demo in the video. When the water spreads across the floor, it looks a little like a puddle of blue dye is diffusing into the water, and it's hard to tell how it would look if the water were more realistically transparent.