Inspired by a picture on the USGS website
Each blue sphere is to scale assuming the Earth is a sphere with a radius of 40 cm (15.74 inches). The largest one is all of the water on Earth. The mid-sized sphere is all fresh liquid water and the smallest is lake and river water only. The white base is a slice of a 40 cm sphere (size of the Earth)
More information here
The version in the photo was printed on a replicator with 0.2 mm layers and 10% infill.
This is the smallest size I was able to print without losing the smallest sphere.
For the two color version use the waterOnEarth800Water and waterOnEarth800Base stl files. Use the waterOnEarthAll.stl file to print a single color version.
The waterAndEarth800.stl file has all three water spheres and the entire Earth to scale. It is much too large to print, but I've included it to help provide some additional perspective on the scale of the Earth relative to the water spheres.