How would I achieve this low poly water effect using cycles?
But the closest I've gotten would be this:
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It looks to me like some kind of mix between glossy and transparent shaders.
The trick is that there seems to be multiple layers of mesh superimposed, allowing objects near the surface to be seen, while nearly completely obscuring the bottom.
One way to go about creating such layers is by first creating a "base mesh". I used a fluid simulation, but for more control you may want to use dyntopo.
Also note the way the water meshes in your example have been sculpted to give the impression of currents; long poly strips where water is flowing straight, and choppier, evenly distributed triangles for more turbulent regions.
I didn't do this, but you may want to.
You can use a Glass Material as a base and set the IOR (indice of refraction) to 1.39336 which is the value for the water at 20°C.
An important thing is also the environment texture setting as the reflection is important.
On this base, you'll probably need to add turbulences effects. But this may be to take into account differently depending on what you want : static image or animation.
After some searches, I found this "swimming pool" effect combi which fits my needs (water is a small part of a big picture). You can adjust the effect by changing glass shader iOR value (which set very strong on illustration).
My goal is to have 1 plane for each water zone with light reflection AND some roughness... This method reaches both goals so it stays animation friendly until water comes at foreground. In this case, I would add some surface displacement as seen in the question picture.
Notes : 1) Unlike in a real pool, light zones are coming from surface reflecting a sun lamp placed at upper front of the camera. 2) Wave texture can be used too but less convincing than musgrave to my opinion.
Possible improvement : Try to transfert effect to the bottom surface, behind water. Would require to recreate glossy effect for water surface.