The amount of bending depends of the refractive index (short for n) of the water. It's a number (without unit) that is 1 (for vacuum) or larger than 1 (for other materials). Different materials have a different refractive index.
The larger the refractive index is, the larger the bending is. For example, let the refractive index of water is less than diamond's. So, if a light ray from air enters water and diamond, the refractive angle (bending of ray) should be larger in diamond comparing to water.
Theres equations to calculate both n or the angles, but I shall not discuss about it right now.
About the rainbow thing, it is more complicated. It doesn't only involve refraction, but even reflection and dispersion, where different color of light ray travels in different speed.
But in short, the bending of light rays in different materials is affected by the difference in the refractive index of the material.
