The magnificent intense blue of Crater Lake is due to its great depth and clarity.
Sunlight is able to penetrate deep into the water.
The depth absorbs many of the longer rays and reflecting the shorter rays of the white light spectrum. Red light is the longest wavelength, thus gets absorbed. Violet and blue light are the shortest wavelengths, thus get scattered and reflected back to us, causing the lake to be its famous Crater Lake blue. The last recorded year in which the lake froze over was in 1949, a very long, cold winter. The surface temperature fluctuates a bit, but the bottom temperature remains quite constant.