PSS is a Procedural Node Shader that can produce snow on any surface right out of the box, and even add snow on top of already existing objects.
You need a lot of samples to see the sparkles in the snow, and it's quite an resource expensive shader, but it can produce beautiful results.
For the setup you need to enable Displacement and Bump in the material settings tab so that the shader can displace the vertexes in your mesh. You also need to subdivide it quite a lot, I would recommend to use the Experimental option coupled with a Subsurface Modifier set to Adaptive mode.
It's simply two different voronoi textures, one with the scale at 40 and the other one at 80. Mixed them up and feed into the displacement. And the surface shader is simply a Glass BDSF.
How did you make the water in the first image? It looks very nice!