Sound travels faster through water. Does light travel faster or slower through water?
When light travels through water, it encounters water molecules which may temporarily absorb and re-emit the light. This takes a small amount of time, which increases the time it takes light to traverse a certain distance in water.
Water is more dense than air. Water molecules are much closer together, which means there's a shorter straight-line path that light can follow before it hits another molecule. In air the molecules are farther apart, so light makes more progress from one molecule to the next.
So the rate at which light propagates through a transparent medium depends on two things: the average amount of time it takes for each molecule to absorb and re-emit the light, and the average distance between molecules.
Simplifying other answers..... (simple is what I like) some of the photons are absorbed by a medium and radiated as heat, the ones that move through water encounter denser atoms, photons are absorbed by atoms are then re-emitted, a photon will hit the electrons surrounding an atom, moving the electon to higher energy levels, then re-emitted returning the electron back to its lower energy levels, and therefore a time delay in this happening, light travels slightly slower through water, it cannot (as far as our physics understand) travel faster than through a vacuum where it encounters no atoms. Water on the other hand has atoms which are denser(closer together) and as a mechanical wave(vibrations of matter) it takes less time to pass vibrations from one to another... one reason sound in space cannot be heard as there are no particles to pass this mechanical wave.
Light travels slower in water.The speed of light is fastest in vaccum followed by air and then any other media which have refractive index greater than.This can be seen by the fact that light bends towards the normal while passing from air to water.
I have an answer but it based on my own ideas and differs from main stream science.
Space is a thing. Light travels through space at C. Light only travels through space and nothing else.
When light travels through water, it is moving through the space between the atoms.
Matter curves the space surrounding it. The space between the atoms of the water is curved.
When light travels through water it is traveling through space that is curved. Light's path through water is a series of curves and therefore light travels a longer distance or travels through more space when moving through a given length of water as compared to a given length of space.
The answer to your question is that light does not travel slower through water than through air, it travels through more space. Light always moves at C relative to space and light only travels through space.
Some of these answers might create an incorrect view of the matter. If an atom or molecule absorbs a photon, that is the end of the photon. That happens in things that are opaque. Water does absorb some photons, but it doesn't re-emit them. Try scuba diving in Hawaii and the brightly-colored fish all look blue/green at 30 feet deep or more, because all the yellow-orange-red light was absorbed. Most things that absorb sunlight just get warm; they don't re-emit visible light unless they are further heated to as hot as flames or red-hot steel, and then they emit the light randomly in all directions.
Light can travel through a vacuum by throwing down an electric field which throws down a magnetic field which throws down an electric field which throws down another magnetic field and so on, typically 10^15 times per second (in visible range). When travelling through a medium, the growth of each electric field is slowed by the presence of other electric fields from the electron shells of atoms. In this case, the light propagates like a physical wave (like the 19th century idea of a sea of ether). In a vacuum, light travels 186,000 miles per second. In water, it only moves at 140,000 miles per second. Researchers using extremely cold Bose-Einstein condensate at less than a degree F above absolute zero make a thick substance through which light waves move as though through syrup, slowing to just a few miles per hour (google "slow light" or "Dr. Hau" for more on that, or see How to slow down light until it stops).
Light travels in a vacuum at 186,282mps. You must remember that any medium will slow lights travel through it. Take the sun for example, a photon is generated at it's core and from the first instant it is trying to get to the surface and get out. This, RANDOM WALK can take anywhere from a few thousand to a million years to reach the surface. Here is a great reading about the difference in light speed through different mediums;