Why is the ocean blue? No, it's not because it reflects the sky

The ocean looks blue mainly because water absorbs sunlight at longer wavelengths far more effectively than at shorter wavelengths, leaving blue light to be reflected and scattered back to the eye. In a glass, water appears colorless because the light path is too short for the absorption effect to build up. A common explanation—ocean blue as a reflection of the sky—doesn’t match measurements: reflections from the sky account for about 2% of the color observed at the water’s surface, while the remainder comes from how water molecules filter the Sun’s spectrum. The article also notes that impurities like sediment or algae can shift lakes toward green, brown or black, erasing the blue physics.