The speed of light in water is approximately 225,000 km per second. While enormously fast, this is notably slower than the speed of light in a vacuum, which is 300,000 km per second.
The degree that light slows down when passing through a medium is described by the index of refraction of that medium. This index measures how much the medium alters the course that the light travels through reflection, refraction, occlusion or other obstruction. Calculating the speed of light in the medium requires dividing the speed of light by the index of refraction. This results in the equation v = c / n, where v is the speed that light travels in the medium, c is the speed of light in a vacuum and n is the index of refraction.