* Air: Electromagnetic waves travel at approximately the speed of light, which is 299,792,458 meters per second (m/s). In air, the speed is very slightly less than this due to the very low density of air, but the difference is negligible for most practical purposes.

* Water: The speed of electromagnetic waves in water is significantly slower than in air. It depends on the frequency of the wave, but it's typically around 225,000,000 m/s. This is about 75% of the speed of light in a vacuum.

Why the difference?

The speed of light changes in different mediums because of the interaction of the electric and magnetic fields of the light wave with the electric charges in the medium. Water molecules have a higher density and polarizability than air molecules, which leads to a greater interaction and a slower propagation speed.