There are only three known blue water lakes in the world. Lake Tahoe (California/Nevada), Lake Geneva (in Wisconsin, USA), and West Lake Okoboji (Iowa), though even these are open to interpretation. Some contend that Lake Superior and Crater Lake are also blue water lakes.
70% of the earth is covered with water, but only 2-3% is fresh, and still even less is drinkable - estimated as only 1% of the world's water ecosystem, according to the National Geographic. Blue water reflects wavelength of light which results in a blue appearance rather than a more common turquoise mixture of blue and green.