Does light travel faster in seawater or in fresh water? Does light travel faster in seawater or in fresh water? Light travels faster in seawater, because the index of refraction is smaller in seawater. Light travels faster in fresh water, because the index of refraction is smaller in fresh water. Light travels faster in fresh water, because the index of refraction is larger in fresh water. Light travels faster in seawater, because the index of refraction is larger in seawater.