Here's why:

* Light slows down when it travels through a denser medium. Water is denser than air, meaning there are more molecules packed together.

* Light interacts with the molecules in the medium. This interaction causes the light to slow down.

* The index of refraction is a measure of how much a medium slows down light. Water has a higher index of refraction than air, indicating that light travels slower in water.

While the speed of light in a vacuum is a constant, the speed of light in different mediums can vary.