A large reason for the water's clarity is because Crater Lake has no inlets or tributaries. Located in southern Oregon, it is completely fed by precipitation such as rain and snow. At a depth of 1,949 feet, Crater Lake is also the deepest lake in the United States. The world's next-clearest lake is Russia's Lake Baikal, which also happens to be the deepest lake on Earth!
It's so clear that scientists can see clearly 142 feet deep into the water. In most lakes, you can't see clearly for more than about 16 feet deep. You can visit Crater Lake, and even dive in it, though to do so you need to carry your heavy diving equipment on foot over a long trail.
Check out this video of divers in Crater Lake. The water is so clear, that it's disorienting for experienced divers.