Water has a faint intrinsic blue color due to the selective absorption of red wavelengths of light, which is a well-established physical fact that requires no academic citation.
The claim that water has an intrinsic blue color is a well-known scientific fact (stemming from vibrational overtone absorption in the red spectrum) that can be verified through direct observation of large bodies of water. Therefore, it is classified as common knowledge, and none of the retrieved papers—which deal with engineering, nanomaterials, and solar cells—are required as citations.