As light travels through water it can give us some information about the content of the water. On calm days the water surface acts almost like a mirror. But look straight down, and the mirror disappears. Instead you see what is under the surface. If you look further away, your viewing angle increases, and the surface reflection becomes stronger. At the same time more of the light from under the surface is reflected back down.
Have you ever put a stick into the water and watched it bend at the surface? You know it's straight, so what you see is an optical illusion. The reason is refraction. Light from under the water is bent as it passes the surface into the air. As a result the underwater part of the stick seems to be in a different place from where it really is.
Refraction can also make things in the water appear larger than they are. It is also the reason why waves often focus sunlight into patterns of light and shadow on the bottom.
Once in the water the light continues to travel downwards, gradually growing weaker. How deep it goes depends on the type of water and on the angle of the sun.
There are two reasons for the loss of light as you go deeper:
Absorption - photons disappear and the energy they contain is turned into heat or used for photosynthesis.
Scattering - photons change direction, but do not disappear. Often the new direction is upwards.
Absorption is what gives water its colour. Open ocean water is usually blue because the blue photons travel furthest before they are absorbed. Water with lots of phytoplankton (microscopic plants) is often bright green. Chlorophyll in the plants absorbs blue light, so now the green photons travel furthest.
Pure or clear water scatters very little light. When the water is clear and clean most of the photons disappear into the deep. That's why clear water seems quite dark when you look straight down.
Small particles in the water (plant cells, decomposed matter, sand and mud) scatter much more light. In this type of water many of the photons change direction and travel upwards. Seen from above when you are looking down this type of water has a much lighter colour.
In sea water, particle scattering and absorption has a BIG effect on your fly and lure fishing. Just as in fog, the scattering and absorption blurs details, and if you were a fish you might only see a short distance ahead. The light also fades faster as you go deeper. In water with lots of small particles, it can be dark at just a few metres depth. Tiny particles in the water scatter light and make everything look blurred and indistinct.