Water - most water assets are done as a flat plane with a texture to simulate ripples. As such it doesn't refract light realistically. Either you need to get an asset with water done as a rippled surface (Rigged Water iRay / Ebano) or create the water surface from scratch by creating a suitable surface in Blender / Hexagon or by using D-formers. Please note that D-formers don't work that well and will only have an effect if the water surface is made up of high resolution mesh.
Down-Sampling - It's just rendering at a higher resolution and the reducing the resolution of the final product. For a 1080p image I render at 3840 x 2160 and then reduce the image size to 1920 x 1080. Whilst each iteration takes 4 times as long, I find I can achieve the same quality result in about 1/10th the iterations. As such the rendering time is about 2.5 times less.
