Blue water lake is a non-scientific term for a lake with exceptionally high-quality water that, when penetrated by sunlight, appears pure blue.
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 SuperiorLake Superior and Crater Lake are also blue water lakes.