the answer is the atmosphere . the water as you all know reflects the images in front of it. in the same way as the atmosphere is blue the sea reflects the blue colour . you might have studied that water is colorless it only reflects the blue color of the atmosphere or the sky
All colors go through the water but they fade out soon. Blue stays in the water the longest because of it's dark color. But eventually, blue fades out the deeper you go and the water becomes black. Scary.
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the color of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation. The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. The result is that the human eye perceives blue when looking toward parts of the sky other than the sun. The color perceived is similar to that obtained by a monochromatic blue of wavelength 474–476 nm mixed with white light, i.e., an unsaturated blue light.