Why is the refractive index of water bigger than that of air?

The refractive index is related to the perceived speed at which light is going in that medium. Water is more dense than air, ie there are more particles occupying a given space. So when light travels through water, there is a higher probability for it to bump into water molecules. Hence its path will deviate from a straight line, it will be a bumpy path. Hence, on a microcoscopic level, the photon is actually travelling a longer bumpy path when in a denser medium, that is why it is perceived to be slower. So the actual speed of light doesn't change. The path it takes in a denser medium is longer than a less dense counterpart.