What is the speed of light in a vacuum?
The speed of light in a vacuum is approximately 299,792,458 meters per second (m/s). This is considered to be the upper limit for the speed of any matter or energy in the universe and is an important constant in physics and astronomy. The exact value of the speed of light is defined by the International System of Units (SI) and is used in many important equations in physics, including those related to electromagnetic waves, special and general relativity, and quantum mechanics.
How does it change in water?
The speed of light slows to around 226,000,000 meters per second in water. It does so because the photons (more correctly the wave describing them) interact with electrons. This interaction results in re-emittance at a delay from the original, causing an apparent slowdown of light in non-vacuous mediums.