When the light travels through the boundary between the air and water, it changes direction. This phenomenon is called refraction. It can be easily observed by filling a glass with water and putting a straw in the glass. The straw will appear to be broken.
The refraction occurs because the speed of light in air is different than the speed of light in water. The speed with which a light wave propagates in a given medium is determined by the electromagnetic properties of this medium, which are different for air in water. In the air, the speed of light is very close to the speed of light in the vacuum, `3*10^8` m/s. In the water, the speed of light is less. Thus, when the wavefront of propagating light hits the boundary between air and water, it sort of breaks and the ray of light travels in a different direction than the original. (Please see the reference link for a more detailed explanation, and also to learn how to determine the angle at which the light refracts.)
The phenomenon of refraction also explains why we can see rainbows. The various colors we can see correspond to different frequencies of the light waves. Normally the light is composed of waves of mixed frequencies and appears white. However, these waves with different frequencies refract at different angles when they pass the boundary between air in water. Then the light waves of different colors become separated, and we can see a rainbow.
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