Why is water transparent ?

Pure water does not absorb any radiation or more precisely saying any part of UV -visible spectrum of light that is from 400nm-700nm and so it doesn't have complimentary Colour which our eyes can sense and it appears colourless to us. In general this the reason behind everything that looks colourless to us.

While relatively small quantities ofwater appear to be colorless, water's tint becomes a deeper blue as the thickness of the observed sample increases.

The blue hue of water is an intrinsic property and is caused by selective absorption and scattering of white light.

However, water is different. When light hits water, there is about a 10% chance (maybe 20%) that the light will actually hit the water, because the water is not dense, but spread apart.

Water is optically transparent. Hence when light rays pass through it, they are not obstructed.

Transparency is simply the property of a medium allowing photons to travel through it. You may have read that before the Cosmic Microwave Background Radiation the universe was 'opaque', and that afterwards it became 'transparent'. Were there human eyes around perhaps?

However refraction does take place when light passes through water from any other medium.

Water is transparent simply because it allows photons of a certain frequency range to travel through it (it does absorb some but not a lot, so the deeper we get the less light reaches there).

The constituents of water is H2O hydrogen is colourless as well as oxygen. Hence water is colourless

As for the evolution of eyes, the Sun's spectrum is something like 5% ultraviolet, 45% visible and 50% infrared or lower (radio, microwaves).

Infrared at low enough frequencies becomes noticed as heat rather than as light, so there's not a very big advantage in seeing it either (although snakes or bats can see it).

The reason we see colors at all is due to electrons in atoms reaching an excited state. When those electrons drop back to their normal states, they emit a certain amount of energy that corresponds to either color (if it happens to be in the visible spectrum) or another type of EM radiation.

Because of water's structure and composition (three atoms: two hydrogens and one oxygen), its excited electrons emit radiation in the ultraviolet region of the spectrum, not in the visible light part (which is actually a tiny piece when compared to the entire thing

Visible light is energy with wavelengths between 390nm and 750nm). Our eyes can only see the visible light portion of the spectrum, and so we are unable to notice any color in water. Its electron absorptions simply occur in a part of the spectrum that we are unable to visibly detect.

Water is transparent to visible radiation or "light," regardless of whether or not our eyes (or any detector, for that matter) can actually see this radiation.

. Whether a substance is "transparent" to light will depend upon the ability of the molecules (or atoms) to absorb and re-emit the wavelengths associated with light, instead of converting the energy into heat. (The presence of molecules that are larger than the wavelength of light can also cause light to scatter, which is why the sky is blue.)

water is only transparent for these wavelengths. It may have contributed to the early evolution of our eyes,

Maybe there is a specific scientific definition of the word transparent but in daily usage the meaning is being danced around. In fact, I just looked up transparent in a Google search and the phrase "transparent blue water" was included in the preferred definition

We can’t see through walls because electrons in the structure absorb visible light. Some light reflects off the wall, but no light gets through. Radio waves – the same stuff as visible light with a longer wavelength – go straight through.

because electrons in a wall don’t absorb or reflect radio waves, due to their shape. Electrons in water act in a similar way to visible light so they don’t absorb or reflect most of the light.

Water is strongly absorbing at most of the wavelengths in the electromagnetic spectrum, but it has a narrow window of transparency which includes the visible spectrum

The window of transparency of water matches the peak emitted intensity of a blackbody radiator at 6000 K, a reasonable model for the output radiation of the Sun. The absorption spectrum of water and the peak of the Sun's radiation are of course physically independent phenomena, but it is fortunate for life on the Earth that they coincide.

The absorption of electromagnetic radiation by water spans a wide range of physical phenomena, characteristic of the general interaction of radiation with matter. It absorbs strongly in the microwave region by excitation of molecular rotations. In the infrared it exhibits strong absorptions from vibrations of the water molecule